How to create a custom JavaScript syntax with Babel

In this post, I'm going to show you how you can create a custom JavaScript syntax with Babel

Overview

Let me show you what we will achieve at the end of this article:

// '@@' makes the function `foo` curried
function @@ foo(a, b, c) {
  return a + b + c;
}
console.log(foo(1, 2)(3)); // 6

We are going to create a curry function syntax @@. The syntax is like the generator function, except you place @@ instead of * in between the function keyword and the function name, eg function @@ name(arg1, arg2).

In this example, you can have partial application with the function foo. Calling foo with the number of parameters less than the arguments required will return a new function of the remaining arguments:

foo(1, 2, 3); // 6

const bar = foo(1, 2); // (n) => 1 + 2 + n
bar(3); // 6

The reason I choose @@ is that you can’t have @ in a variable name, so [email protected]@foo(){} is still a valid syntax. And the “operator” @ is used for decorator functions but I wanted to use something entirely new, thus @@.

To achieve this, we are going to:

• Fork the babel parser
• Create a custom babel transformation plugin

Sounds impossible 😨?

Don’t worry, I will guide you through every step. Hopefully, at the end of this article, you will be the babel master amongst your peers. 🤠

Fork the babel

Head over to babel’s Github repo, click the “Fork” button located at the top left of the page.

If this is your first time forking a popular open-source project, congratulations!

Clone your forked babel to your local workspace and set it up:

$ git clone https://github.com/tanhauhau/babel.git

# set up
$ cd babel
$ make bootstrap
$ make build

Meanwhile, let me briefly walk you through how the babel repository is organised.

Babel uses a monorepo structure, all the packages, eg: @babel/core, @babel/parser, @babel/plugin-transform-react-jsx, etc are in the packages/ folder:

- doc
- packages
  - babel-core
  - babel-parser
  - babel-plugin-transform-react-jsx
  - ...
- Gulpfile.js
- Makefile
- ...

Trivia: Babel uses Makefile for automating tasks. For build task, such as make build, it will use Gulp as the task runner.

Crash Course on Parsing Code to AST

Before we proceed, if you are unfamiliar with parsers and Abstract Syntax Tree (AST), I highly recommend to checkout Vaidehi Joshi’s Leveling Up One’s Parsing Game With ASTs.

To summarise, this is what happened when babel is parsing your code:

• Your code as a string is a long list of characters: f, u, n, c, t, i, o, n, , @, @, f, ...
• The first step is called tokenization, where babel scans through each character and creates tokens, like function, @@, foo, (, a, ...
• The tokens then pass through a parser for Syntax analysis, where babel creates an AST based on JavaScript language specification.

If you want to learn more in-depth on compilers in general, Robert Nystrom’s Crafting Interpreters is a gem.

Don’t get scared of by the word compiler, it is nothing but parsing your code and generate XXX out of it. XXX could be machine code, which is the compiler most of us have in mind; XXX could be JavaScript compatible with older browsers, which is the case for Babel.

Our custom babel parser

The folder we are going to work on is packages/babel-parser/:

- src/
  - tokenizer/
  - parser/
  - plugins/
    - jsx/
    - typescript/
    - flow/
    - ...
- test/

We’ve talked about tokenization and parsing, now it’s clear where to find the code for each process. plugins/ folder contains plugins that extend the base parser and add custom syntaxes, such as jsx and flow.

Let’s do a Test-driven development (TDD). I find it easier to define the test case then slowly work our way to “fix” it. It is especially true in an unfamiliar codebase, TDD allows you to “easily” point out code places you need to change.

packages/babel-parser/test/curry-function.js

import { parse } from '../lib';

function getParser(code) {
  return () => parse(code, { sourceType: 'module' });
}

describe('curry function syntax', function() {
  it('should parse', function() {
    expect(getParser(`function @@ foo() {}`)()).toMatchSnapshot();
  });
});

You can run TEST_ONLY=babel-parser TEST_GREP="curry function" make test-only to run tests for babel-parser and see your failing case:

SyntaxError: Unexpected token (1:9)

at Parser.raise (packages/babel-parser/src/parser/location.js:39:63)
at Parser.raise [as unexpected] (packages/babel-parser/src/parser/util.js:133:16)
at Parser.unexpected [as parseIdentifierName] (packages/babel-parser/src/parser/expression.js:2090:18)
at Parser.parseIdentifierName [as parseIdentifier] (packages/babel-parser/src/parser/expression.js:2052:23)
at Parser.parseIdentifier (packages/babel-parser/src/parser/statement.js:1096:52)

If you find scanning through all the test cases takes time, you can directly call jest to run the test:

BABEL_ENV=test node_modules/.bin/jest -u packages/babel-parser/test/curry-function.js

Our parser found 2 seemingly innocent @ tokens at a place where they shouldn’t be present.

How do I know that? Let’s start the watch mode, make watch, wear our detective cap 🕵️‍ and start digging!

Tracing the stack trace, led us to packages/babel-parser/src/parser/expression.js where it throws this.unexpected().

Let us add some console.log:

packages/babel-parser/src/parser/expression.js

parseIdentifierName(pos: number, liberal?: boolean): string {
  if (this.match(tt.name)) {
    // ...
  } else {
    console.log(this.state.type); // current token
    console.log(this.lookahead().type); // next token
    throw this.unexpected();
  }
}

As you can see, both tokens are @ token:

TokenType {
  label: '@',
  // ...
}

How do I know this.state.type and this.lookahead().type will give me the current and the next token?

Well, I’ll explained them later

Let’s recap what we’ve done so far before we move on:

• We’ve written a test case for babel-parser
• We ran make test-only to run the test case
• We’ve started the watch mode via make watch
• We’ve learned about parser state, and console out the current token type, this.state.type

Here’s what we are going to do next:

If there’s 2 consecutive @, it should not be separate tokens, it should be a @@ token, the new token we just defined for our curry function

A new token: ’@@’

Let’s first look at where a token type is defined: packages/babel-parser/src/tokenizer/types.js.

Here you see a list of tokens, so let’s add our new token definition in as well:

packages/babel-parser/src/tokenizer/types.js

export const types: { [name: string]: TokenType } = {
  // ...
  at: new TokenType('@'),
  atat: new TokenType('@@'),};

Next, let’s find out where the token gets created during tokenization. A quick search on tt.at within babel-parser/src/tokenizer lead us to packages/babel-parser/src/tokenizer/index.js

Well, token types are import as tt throughout the babel-parser.

Let’s create the token tt.atat instead of tt.at if there’s another @ succeed the current @:

packages/babel-parser/src/tokenizer/index.js

getTokenFromCode(code: number): void {
  switch (code) {
    // ...
    case charCodes.atSign:
      // if the next character is a `@`      if (this.input.charCodeAt(this.state.pos + 1) === charCodes.atSign) {        // create `tt.atat` instead        this.finishOp(tt.atat, 2);      } else {        this.finishOp(tt.at, 1);      }      return;    // ...
  }
}

If you run the test again, you will see that the current token and the next token has changed:

// current token
TokenType {
  label: '@@',
  // ...
}

// next token
TokenType {
  label: 'name',
  // ...
}

Yeah! It looks good and lets move on.

The new parser

Before we move on, let’s inspect how generator functions are represented in AST:

As you can see, a generator function is represented by the generator: true attribute of a FunctionDeclaration.

Similarly, we can add a curry: true attribute of the FunctionDeclaration too if it is a curry function:

We have a plan now, let’s implement it.

A quick search on “FunctionDeclaration” leads us to a function called parseFunction in packages/babel-parser/src/parser/statement.js, and here we find a line that sets the generator attribute, let’s add one more line:

packages/babel-parser/src/parser/statement.js

export default class StatementParser extends ExpressionParser {
  // ...
  parseFunction<T: N.NormalFunction>(
    node: T,
    statement?: number = FUNC_NO_FLAGS,
    isAsync?: boolean = false
  ): T {
    // ...
    node.generator = this.eat(tt.star);
    node.curry = this.eat(tt.atat);  }
}

If you run the test again, you will be amazed that it passed!

PASS  packages/babel-parser/test/curry-function.js
  curry function syntax
    ✓ should parse (12ms)

That’s it? How did we miraculously fix it?

I am going to briefly explain how parsing works, and in the process hopefully, you understood what that one-liner change did.

How parsing works

With the list of tokens from the tokenizer, the parser consumes the token one by one and constructs the AST. The parser uses the language grammar specification to decide how to use the tokens, which token to expect next.

The grammar specification looks something like this:

...
ExponentiationExpression -> UnaryExpression
                            UpdateExpression ** ExponentiationExpression
MultiplicativeExpression -> ExponentiationExpression
                            MultiplicativeExpression ("*" or "/" or "%") ExponentiationExpression
AdditiveExpression       -> MultiplicativeExpression
                            AdditiveExpression + MultiplicativeExpression
                            AdditiveExpression - MultiplicativeExpression
...

It explains the precedence of each expressions/statements. For example, an AdditiveExpression is made up of either:

• a MultiplicativeExpression, or
• an AdditiveExpression followed by + operator token followed by MultiplicativeExpression, or
• an AdditiveExpression followed by - operator token followed by MultiplicativeExpression.

So if you have an expression 1 + 2 * 3, it will be like:

(AdditiveExpression "+" 1 (MultiplicativeExpression "*" 2 3))

instead of

(MultiplicativeExpression "*" (AdditiveExpression "+" 1 2) 3)

With these rules, we translate them into parser code:

class Parser {
  // ...
  parseAdditiveExpression() {
    const left = this.parseMultiplicativeExpression();
    // if the current token is `+` or `-`
    if (this.match(tt.plus) || this.match(tt.minus)) {
      const operator = this.state.type;
      // move on to the next token
      this.nextToken();
      const right = this.parseMultiplicativeExpression();

      // create the node
      this.finishNode(
        {
          operator,
          left,
          right,
        },
        'BinaryExpression'
      );
    } else {
      // return as MultiplicativeExpression
      return left;
    }
  }
}

As you can see here, the parser is recursively in nature, and it goes from the lowest precedence to the highest precedence expressions/statements. Eg: parseAdditiveExpression calls parseMultiplicativeExpression, which in turn calls parseExponentiationExpression, which in turn calls … . This recursive process is called the Recursive Descent Parsing.

this.eat, this.match, this.next

If you have noticed, in my examples above, I used some utility function, such as this.eat, this.match, this.next, etc. These are babel parser’s internal functions, yet they are quite ubiquitous amongst parsers as well:

• this.match returns a boolean indicating whether the current token matches the condition

• this.next moves the token list forward to point to the next token

• this.eat return what this.match returns and if this.match returns true, will do this.next

  • this.eat is commonly used for optional operators, like * in generator function, ; at the end of statements, and ? in typescript types.

• this.lookahead get the next token without moving forward to make a decision on the current node

If you take a look again the parser code we just changed, it’s easier to read it in now.

packages/babel-parser/src/parser/statement.js

export default class StatementParser extends ExpressionParser {
  parseStatementContent(/* ...*/) {
    // ...
    // NOTE: we call match to check the current token
    if (this.match(tt._function)) {
      this.next();
      // NOTE: function statement has a higher precendence than a generic statement
      this.parseFunction();
    }
  }
  // ...
  parseFunction(/* ... */) {
    // NOTE: we call eat to check whether the optional token exists
    node.generator = this.eat(tt.star);
    node.curry = this.eat(tt.atat);    node.id = this.parseFunctionId();
  }
}

I know I didn’t do a good job explaining how a parser works. Here are some resources that I learned from, and I highly recommend them:

• Crafting Interpreters by Robert Nystrom
• Free Udacity course: “Compilers: Theory and Practice”, offered by Georgia Tech

Side Note: You might be curious how am I able to visualize the custom syntax in the Babel AST Explorer, where I showed you the new “curry” attribute in the AST.

That’s because I’ve added a new feature in the Babel AST Explorer where you can upload your custom parser!

If you go to packages/babel-parser/lib, you would find the compiled version of your parser and the source map. Open the drawer of the Babel AST Explorer, you will see a button to upload a custom parser. Drag the packages/babel-parser/lib/index.js in and you will be visualizing the AST generated via your custom parser!

Our babel plugin

With our custom babel parser done, let’s move on to write our babel plugin.

But maybe before that, you may have some doubts on how are we going to use our custom babel parser, especially with whatever build stack we are using right now?

Well, fret not. A babel plugin can provide a custom parser, which is documented on the babel website

babel-plugin-transformation-curry-function.js

import customParser from './custom-parser';

export default function ourBabelPlugin() {
  return {
    parserOverride(code, opts) {
      return customParser.parse(code, opts);
    },
  };
}

Since we forked out the babel parser, all existing babel parser options or built-in plugins will still work perfectly.

With this doubt out of the way, let see how we can make our curry function curryable? (not entirely sure there’s such word)

Before we start, if you have eagerly tried to add our plugin into your build system, you would notice that the curry function gets compiled to a normal function.

This is because, after parsing + transformation, babel will use @babel/generator to generate code from the transformed AST. Since the @babel/generator has no idea about the new curry attribute we added, it will be omitted.

If one day curry function becomes the new JavaScript syntax, you may want to make a pull request to add one more line here!

Ok, to make our function curryable, we can wrap it with a currying helper higher-order function:

function currying(fn) {
  const numParamsRequired = fn.length;
  function curryFactory(params) {
    return function (...args) {
      const newParams = params.concat(args);
      if (newParams.length >= numParamsRequired) {
        return fn(...newParams);
      }
      return curryFactory(newParams);
    }
  }
  return curryFactory([]);
}

If you want to learn how to write a currying function, you can read this Currying in JS

So when we transform our curry function, we can transform it into the following:

// from
function @@ foo(a, b, c) {
  return a + b + c;
}

// to
const foo = currying(function foo(a, b, c) {
  return a + b + c;
})

Let’s first ignore function hoisting in JavaScript, where you can call foo before it is defined.

If you have read my step-by-step guide on babel transformation, writing this transformation should be manageable:

babel-plugin-transformation-curry-function.js

export default function ourBabelPlugin() {
  return {
    // ...
    visitor: {      FunctionDeclaration(path) {        if (path.get('curry').node) {          // const foo = curry(function () { ... });          path.node.curry = false;          path.replaceWith(            t.variableDeclaration('const', [              t.variableDeclarator(                t.identifier(path.get('id.name').node),                t.callExpression(t.identifier('currying'), [                  t.toExpression(path.node),                ])              ),            ])          );        }      },    },  };
}

The question is how do we provide the currying function?

There are 2 ways:

1. Assume currying has been declared in the global scope.

Basically, your job is done here.

If currying is not defined, then when executing the compiled code, the runtime will scream out “currying is not defined”, just like the “regeneratorRuntime is not defined”.

So probably you have to educate the users to install currying polyfills in order to use your babel-plugin-transformation-curry-function.

2. Use the @babel/helpers

You can add a new helper to @babel/helpers, which of course you are unlikely to merge that into the official @babel/helpers, so you would have to figure a way to make @babel/core to resolve to your @babel/helpers:

package.json

{
  "resolutions": {
    "@babel/helpers": "7.6.0--your-custom-forked-version",
  }
}

Adding a new helper function into @babel/helpers is very easy.

Head over to packages/babel-helpers/src/helpers.js and add a new entry:

helpers.currying = helper("7.6.0")`
  export default function currying(fn) {
    const numParamsRequired = fn.length;
    function curryFactory(params) {
      return function (...args) {
        const newParams = params.concat(args);
        if (newParams.length >= numParamsRequired) {
          return fn(...newParams);
        }
        return curryFactory(newParams);
      }
    }
    return curryFactory([]);
  }
`;

The helper tag function specifies the @babel/core version required. The trick here is to export default the currying function.

To use the helper, just call the this.addHelper():

// ...
path.replaceWith(
  t.variableDeclaration('const', [
    t.variableDeclarator(
      t.identifier(path.get('id.name').node),
      t.callExpression(this.addHelper("currying"), [
        t.toExpression(path.node),
      ])
    ),
  ])
);

The this.addHelper will inject the helper at the top of the file if needed, and returns an Identifier to the injected function.

Closing Note

We’ve seen how we can modify the babel parser function, write our own babel transform plugin (which was brief mainly because I have a detailed cover in previous post, a brief touch on @babel/generator and also how we can add helper functions via @babel/helpers.

Along the way, we had a crash course on how a parser works, which I will provide the links to further reading at the bottom.

The steps we’ve gone through above is similar to part of the TC39 proposal process when defining a new JavaScript specification. When proposing a new specification, the champion of the proposal usually write polyfills or forked out babel to write proof-of-concept demos. As you’ve seen, forking a parser or writing polyfills is not the hardest part of the process, but to define the problem space, plan and think through the use cases and edge cases, and gather opinions and suggestions from the community. To this end, I am grateful to the proposal champion, for their effort in pushing the JavaScript language forward.

Finally, if you want to see the code we’ve done so far in a full picture, you can check it out from Github.

In this article on what is JavaScript, we will learn the basic concepts of JavaScript.

After decades of improvement, JavaScript has become one of the most popular programming languages of all time. It all started in the year 1995 when Brendan Eich created JavaScript in a span of 10 days. Since then, it has seen multiple versions, updates and has grown to the next level.

Here’s a list of topics that I’ll be covering in this blog:

1. What is JavaScript
2. What can JavaScript do?
3. JavaScript Frameworks
4. The Big Picture: HTML, CSS & JavaScript
5. Benefits of JavaScript
6. Fundamentals of JavaScript
   VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case

What is JavaScript?

JavaScript is a high level, interpreted, programming language used to make web pages more interactive.

Have you ever thought that your website is missing something? Maybe it’s not engaging enough or it’s not as creative as you want it to be. JavaScript is that missing piece which can be used to enhance web pages, applications, etc to provide a more user-friendly experience.

What is JavaScript?

JavaScript is the language of the web, it is used to make the web look alive by adding motion to it. To be more precise, it’s a programming language that let’s you implement complex and beautiful things/design on web pages. When you notice a web page doing more than just sit there and gawk at you, you can bet that the web page is using JavaScript.

Feature of JavaScript

Scripting language and not Java: In fact, JavaScript has nothing to do with Java. Then why is it called “Java” Script? When JavaScript was first released it was called Mocha, it was later renamed to LiveScript and then to JavaScript when Netscape (founded JavaScript) and Sun did a license agreement. Object-based scripting language which supports polymorphism, encapsulation and to some extent inheritance as well.**Interpreted language: **It doesn’t have to be compiled like Java and C which require a compiler.JavaScript runs in a browser: You can run it on Google Chrome, Internet Explorer, Safari, etc. JavaScript can execute not only in the browser but also on the server and any device which has a JavaScript Engine.

What is JavaScript – Stackoverflow stats

Currently, we have 100s of programming languages and every day new languages are being created. Among these are few powerful languages that bring about big changes in the market and JavaScript is one of them.

JavaScript has always been on the list of popular programming languages. According to StackOverflow, for the 6th year in a row, JavaScript has remained the most popular and commonly used programming language.

What can JavaScript do?

JavaScript is mainly known for creating beautiful web pages & applications. An example of this is Google Maps. If you want to explore a specific map, all you have to do is click and drag with the mouse. And what sort of language could do that? You guessed it! It’s JavaScript.JavaScript can also be used in smart watches. An example of this is the popular smartwatch maker called Pebble. Pebble has created Pebble.js which is a small JavaScript Framework that allows a developer to create an application for the Pebble line of watches in JavaScript.

What is JavaScript – Applications of JavaScript
Most popular websites like Google, Facebook, Netflix, Amazon, etc make use of JavaScript to build their websites.Among things like mobile applications, digital art, web servers and server applications, JavaScript is also used to make Games. A lot of developers are building small-scale games and apps using JavaScript.## JavaScript Frameworks

One major reason for the popularity of JavaScript is the JavaScript Frameworks. Here’s a brief introduction of the most trending JavaScript frameworks :

1. AngularJS is Google’s web development framework which provides a set of modern development and design features for rapid application development.

2. ReactJS is another top JavaScript framework mainly maintained by Facebook and it’s behind the User Interface of Facebook and Instagram, showing off its efficiency in maintaining such high traffic applications.

What is JavaScript – JavaScript Frameworks

1. MeteorJS is mainly used for providing back-end development. Using JavaScript on the back-end to save time and build expertise is one of the major ideas behind Meteor.

2. jQuery can be used when you want to extend your website and make it more interactive. Companies like Google, WordPress and IBM rely on jQuery.

The Big Picture: HTML, CSS & JavaScript

Anyone familiar with JavaScript knows that it has something to do with HTML and CSS. But what is the relationship between these three? Let me explain this with an analogy.

What is JavaScript – HTML, CSS and JavaScript

Think of HTML (HyperText Markup Language) as the skeleton of the web. It is used for displaying the web.

On the other hand, CSS is like our clothes. We put on fashionable clothes to look better. Similarly, the web is quite stylish as well. It uses CSS which stands for Cascading Style Sheets for styling purpose.

Then there is JavaScript which puts life into a web page. Just like how kids move around using the skateboard, the web also motions with the help of JavaScript.

Benefits of JavaScript

There has to be a reason why so many developers love working on JavaScript. Well, there are several benefits of using JavaScript for developing web applications, here’s a few benefits:

It’s easy to learn and simple to implement. It is a weak-type programming language unlike the strong-type programming languages like Java and C++, which have strict rules for coding.

It’s all about being fast in today’s world and since JavaScript is mainly a client-side programming language, it is very fast because any code can run immediately instead of having to contact the server and wait for an answer.

Rich set of frameworks like AngularJS, ReactJS are used to build web applications and perform different tasks.

**Builds interactive websites: **We all get attracted to beautifully designed websites and JavaScript is the reason behind such attractive websites and applications.

JavaScript is an interpreted language that does not require a compiler because the web interprets JavaScript. All you need is a browser like Google Chrome or Internet Explorer and you can do all sorts of stuff in the browser.

JavaScript is platform independent and it is supported by all major browsers like Internet Explorer, Google Chrome, Mozilla Firefox, Safari, etc.

JavaScript Fundamentals

In this What is JavaScript blog, we’ll cover the following basic fundamentals of JavaScript
VariablesConstantsData TypesObjectsArraysFunctionsConditional statementsLoopsSwitch case## Variables

Variable is a name given to a memory location which acts as a container for storing data temporarily. They are nothing but reserved memory locations to store values.

What is JavaScript – Variables

To declare a variable in JavaScript use the ‘let’ keyword. For example:

let age;
age=22;

In the above example, I’ve declared a variable ‘age’ by using the ‘let’ keyword and then I’ve stored a value (22) in it. So here a memory location is assigned to the ‘age’ variable and it contains a value i.e. ’22’.

Constants

Constants are fixed values that don’t change during execution time.

To declare a constant in JavaScript use the ‘const’ keyword. For example:

const mybirthday;
mybirthday='3rd August'; 

Data types

You can assign different types of values to a variable such as a number or a string. In JavaScript, there are two categories of data types :

What is JavaScript – Data Types

Objects

An object is a standalone entity with properties and types and it is a lot like an object in real life. For example, consider a girl, whose name is Emily, age is 22 and eye-color is brown. In this example the object is the girl and her name, age and eye-color are her properties.

What is JavaScript – Objects example

Objects are variables too, but they contain many values, so instead of declaring different variables for each property, you can declare an object which stores all these properties.

To declare an object in JavaScript use the ‘let’ keyword and make sure to use curly brackets in such a way that all property-value pairs are defined within the curly brackets. For example:

let girl= {
name: 'Emily',
age: 22,
eyeColour: 'Brown'
};

In the above example, I’ve declared an object called ‘girl’ and it has 3 properties (name, age, eye colour) with values (Emily, 22, Brown).

Arrays

An array is a data structure that contains a list of elements which store multiple values in a single variable.

For example, let’s consider a scenario where you went shopping to buy art supplies. The list of items you bought can be put into an array.

What is JavaScript – Arrays example

To declare an array in JavaScript use the ‘let’ keyword with square brackets and all the array elements must be enclosed within them. For example:

let shopping=[];
shopping=['paintBrush','sprayPaint','waterColours','canvas'];

In the above example I’ve declared an array called ‘shopping’ and I’ve added four elements in it.

Also, array elements are numbered from zero. For example this is how you access the first array element:

shopping[0];		

Functions

A function is a block of organised, reusable code that is used to perform single, related action.

Let’s create a function that calculates the product of two numbers.

To declare a function in JavaScript use the ‘function’ keyword. For example:

function product(a, b) {
return a*b;
}

In the above example, I’ve declared a function called ‘product’ and I’ve passed 2 parameters to this function, ‘a’ and ‘b’ which are variables whose product is returned by this function. Now, in order to call a function and pass a value to these parameters you’ll have to follow the below syntax:

product(8,2);

In the above code snippet I’m calling the product function with a set of values (8 & 2). These are values of the variables ‘a’ and ‘b’ and they’re called as arguments to the function.

Conditional statements – if

Conditional statement is a set of rules performed if a certain condition is met. The ‘if’ statement is used to execute a block of code, only if the condition specified holds true.

What is JavaScript – if flowchart

To declare an if statement in JavaScript use the ‘if’ keyword. The syntax is:

if(condition) {
statement;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[2]) {
console.log('Correct!');
}

In the above example I’ve defined an array of numbers and then I’ve defined an if block. Within this block is a condition and a statement. The condition is ‘(numbers[0]==numbers[2])’ and the statement is ‘console.log(‘Correct!’)’. If the condition is met, only then the statement will be executed.

Conditional statements- Else if

Else statement is used to execute a block of code if the same condition is false.

What is JavaScript – Else-if flowchart

The syntax is:

if(condition) {
statement a;
}
else (condition) {
statement b;
}

Now let’s look at an example:

let numbers=[1,2,1,2,3,2,3,1];
if(numbers[0]==numbers[4] {
console.log("Correct!");
}
else {
console.log("Wrong, please try again");
}

In the above example, I’ve defined an if block as well as an else block. So if the conditions within the if block holds false then the else block gets executed. Try this for yourself and see what you get!

**Loops **

Loops are used to repeat a specific block until some end condition is met. There are three categories of loops in JavaScript :

1. while loop
2. do while loop
3. for loop

While loop

While the condition is true, the code within the loop is executed.

What is JavaScript – while loop flowchart

The syntax is:

while(condition) {
loop code;
}

Now let’s look at an example:

let i=0;
while(i < 5) {
console.log("The number is " +i);
i++;
}

In the above example, I’ve defined a while loop wherein I’ve set a condition. As long as the condition holds true, the while loop is executed. Try this for yourself and see what you get!

Do while loop

This loop will first execute the code, then check the condition and while the condition holds true, execute repeatedly.

What is JavaScript – Do while loop flowchart

Refer the syntax to better understand it:

do {
loop code;
} while(condition);

This loop executes the code block once before checking if the condition is true, then it will repeat the loop as long as the condition holds true.

Now let’s look at an example:

do {
console.log("The number is " +i);
i++;
}
while(i > 5);

The above code is similar to the while loop code except, the code block within the do loop is first executed and only then the condition within the while loop is checked. If the condition holds true then the do loop is executed again.

For loop

The for loop repeatedly executes the loop code while a given condition is TRUE. It tests the condition before executing the loop body.

What is JavaScript – for loop flowchart

The syntax is:

for(begin; condition; step) {
loop code;
}

In the above syntax:

• begin statement is executed one time before the execution of the loop code
• condition defines the condition for executing the loop code
• step statement is executed every time after the code block has been executed

For example:

for (i=0;i<5;i++) {
console.log("The number is " +i);
}

In the above example, I’ve defined a for loop within which I’ve defined the begin, condition and step statements. The begin statement is that ‘i=0’. After executing the begin statement the code within the for loop is executed one time. Next, the condition is checked, if ‘i<5’ then, the code within the loop is executed. After this, the last step statement (i++) is executed. Try this and see what you get!

Switch Case

The switch statement is used to perform different actions based on different conditions.

What is JavaScript – Switch case flowchart

Let’s look at the syntax for switch case:

switch(expression) {
case 1:
code block 1
break;
case 2:
code block 2
break;
default:
code block 3
break;
}

How does it work?

• Switch expression gets evaluated once
• Value of the expression is compared with the values of each case
• If there is a match, the associated block of code is executed

Let’s try this with an example:

let games='football';
switch(games) {
case "throwball":
console.log("I dislike throwball!");
break;
case "football":
console.log("I love football!");
break;
case "cricket":
console.log("I'm a huge cricket fan!");
break;
default:
console.log("I like other games");
break;
}

In the above example the switch expression is ‘games’ and the value of games is ‘football’. The value of ‘games’ is compared with the value of each case. In this example it is compared to ‘throwball’, ‘cricket’ and ‘football’. The value of ‘games’ matches with the case ‘football’, therefore the code within the ‘football’ case is executed. Try this for yourself and see what you get!

With this, we come to the end of this blog. I hope you found this blog informative and I hope you have a basic understanding of JavaScript. In my next blog on JavaScript I’ll be covering in-depth concepts, so stay tuned.

Also, check out our video on JavaScript Fundamentals if you want to get started as soon as possible and don’t forget to leave a comment if you have any doubt and also, let us know whether you’d want us to create more content on JavaScript. We are listening!

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Classes in JavaScript are a special syntax for its prototypical inheritance model that resembles class based inheritance in other object oriented languages. Classes are just special functions that can be declared to resembles classes in other languages. In JavaScript, we can have class declarations and class expressions, because they are just functions. So like all other functions, there are function declarations and function expressions. Classes serve a templates to create new objects.

Defining Classes

To declare a class, or make a class declaration, we use the class keyword to do so. For example, to declare a simple class, we can write:

class Person{
  constructor(firstName, lastName) {
    this.firstName= firstName;
    this.lastName = lastName;
  }
}

Class declarations aren’t hoisted so they can used before they are defined in the code, as the JavaScript interpreter will not automatically pull them up to the top. So the class above won’t work before it’s defined in the code like the following:

const person = new Person('John', 'Smith');
class Person{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

We will get a ReferenceError if we run the code above.

We can also define a class by a class expression, which is an alternative syntax for defining a class. They can be named or unnamed. We can also assign a class to a variable like we do with functions. If we do that, we can reference the class by its name. For example, we can define:

let Person = class {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

To get the name of the unnamed classes above, we can get the name with the name property, like so:

console.log(Person.name);

We can also undefined a named class like the following:

let Person = class Person2{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

Then to get the name of the class, we can use the name property again. So we if we write:

console.log(Person.name)

we get Person2 logged.

The class body is defined with curly brackets. We define the class members inside the brackets. The body of the class is executed in strict mode, so everything defined in strict mode applies to the definition of a class, so we can’t define variables with out some keyword before it like var , let or const , and many other rules apply when you define a class. Classes in JavaScript also have a constructor method that lets us set fields when the object is instantiated with a class . Each class can only have one constructor method in it. If there’s more than one, then SyntaxError will be thrown. A constructor have to also call the super method to call the constructor of the super class inside if it the class extends a parent class.

Methods that aren’t declared static constitutes of the prototypical methods of the class. They are called after an object has been created by using the new keyword. For example, the following class have only prototypical methods:

class Person{
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  get fullName(){
    return `${this.firstName} ${this.lastName}`  
  }
  sayHi(){
    return `Hi, ${this.firstName} ${this.lastName}`
  }
}

In the Person class above, fullName and sayHi are prototypical methods. They are called like this:

const person = new Person('Jane', 'Smith');
person.fullName() // 'Jane Smith'

Static methods are methods that can be called without creating an object from the class using the new keyword. For instance, we can have something like the following:

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  get fullName() {
    return `${this.firstName} ${this.lastName}`
  }
  sayHi() {
    return `Hi, ${this.firstName} ${this.lastName}`
  }
  static personCount() {
    return 3;
  }
}

We can call the personCount function without using the new keyword to create an instance of the class. So if we write:

Person.personCount

We get 3 returned.

The this value inside prototypical methods will be the value of the object. For static methods the value of this has the class that the static method is in as the value.

Getters and Setters

JavaScript classes can have getters and setter functions. Getters, as the name suggests, is a method that lets us get some data from a class. Setters are methods that gives us the ability to set some fields of the class. We denote getter functions with the get keyword and setters with the set keyword. For example, we can write a class that has getters and setters like the following:

class Person {
  constructor(firstName, lastName) {
    this._firstName = firstName;
    this._lastName = lastName;
  }
  get fullName() {
    return `${this.firstName} ${this.lastName}`
  }
  get firstName() {
    return this._firstName
  }
  get lastName() {
    return this._lastName
  }
  sayHi() {
    return `Hi, ${this.firstName} ${this.lastName}`
  }
  set firstName(firstName) {
    this._firstName = firstName;
  }
  set lastName(lastName) {
    this._lastName = lastName;
  }
}

Then when we use the new keyword to construct a Person object, we can use them in the following way:

const person = new Person('Jane', 'Smith');
person.firstName = 'John';
person.lastName = 'Doe';
console.log(person.firstName, person.lastName)

Since we have the getter and setter functions, we can use them to set the data directly to set the data for firstName and lastName of the Person class. In the setter functions, which start with the keyword set , what we assign to them get passed into the parameters and set in the member of the class. In the getter functions, which are denote by get we return the member values so that we can use them.

JavaScript Inheritance

In JavaScript, we can create classes where the properties can be included in the properties of a child class.

So, we can have a high-level class that contains the properties that are common to all the child classes, and the child class can have its own special properties that are not in any other classes.

For example, if we have an Animal class with the common properties and methods, like name and the eat method, then the Bird class can just inherit the common properties in the Animal class. They don’t have to be defined in the Bird class again.

We can write the following to do inheritance in JavaScript:

class Animal {
  constructor(name) {
    this.name = name;
  }
  eat() {
    console.log('eat');
  }
}
class Bird extends Animal {
  constructor(name, numWings) {
    super(name);
    this.numWings = numWings;
  }
}
const bird = new Bird('Joe', 2);
console.log(bird.name)
bird.eat();

In the example above, we have the parent class, Animal, that has the eat method, which all classes that extends from Animal will have, so they don’t have to define eat again.

We have the Bird class which extends the Animal class. Note that in the constructor of the Bird class, we have the super() function call to call the parent’s class constructor to populate the properties of the parent class in addition to the properties of the child class.

Classes cannot extend regular objects, which cannot be constructed with the new keyword. If we want to inherit from a regular object, we have to use the Object.setPrototypeOf function to set a class to inherit from a regular object. For example:

const Animal = {
  eat() {
    console.log(`${this.name} eats`);
  }
};
class Cat{
  constructor(name) {
    this.name = name;
  }
}
class Chicken{
  constructor(name) {
    this.name = name;
  }
}
Object.setPrototypeOf(Cat.prototype, Animal);
Object.setPrototypeOf(Chicken.prototype, Animal);
let cat = new Cat('Bob');
let chicken = new Chicken('Joe');
cat.eat();
chicken.eat();

If we run the example code above, we have see Bob eats and Joe eats logged because we have inherited the eat function from the Animal object.

this Keyword

The this keyword allows us to access the current object’s properties inside an object, unless you’re using arrow functions.

As we can see from the above example, we can get the properties of the instance of the child and the parent class in the object.

Mixins

We can use mixins to do multiple inheritance in JavaScript. Mixins are templates for creating classes. We need mixins to do multiple inheritance because JavaScript classes can only inherit from one super class, so multiple inheritance isn’t possible.

For example, if we have a base class, we can define mixins to incorporate the members from multiple classes into one by composing the mixins by calling one and then pass the returned result into the next one as the argument, an so on, like so:

class Base {
  baseFn() {
    console.log('baseFn called');
  }
}
let classAMixin = Base => class extends Base {
  a() {
    console.log('classAMixin called');
  }
};
let classBMixin = Base => class extends Base {
  b() {
    console.log('classBMixin called');
  }
};
class Bar extends classAMixin(classBMixin(Base)) {}
const bar = new Bar();
bar.baseFn()
bar.a()
bar.b()

In the code above, we have the Base class which we pass into the classBMixin to get the b function into the Base class, then we call the classAMixin by passing in the result of classBMixin(Base) into the argument of the classAMixin to return the a function from classAMixin into the Base class and then return the whole class with all the functions from all the classes incorporated into one.

If we call all the functions above like we did by creating an instance of the Bar object and then call the baseFn , a and b functions, then we get:

baseFn called
classAMixin called
classBMixin called

This means that we have all the functions from the mixins incorporated into the new Bar class.

In JavaScript, classes are just syntactic sugar to make the prototypical inheritance of JavaScript clearer by letting us structure the code in a way that’s more like typical inheritance class based object oriented inheritance pattern. This means that we write classes to and use the new keyword to create objects from the classes, but underneath the syntactic sugar, we are still using prototypical inheritance to extend objects. We can extend classes from objects and we can also use mixins to do multiple inheritance in of JavaScript classes.