JavaScript is a complex language. If you’re a JavaScript developer at any level, it’s important to understand its foundational concepts. This article tackles 12 concepts that are critical for any JS developer to understand, but in no way represents the full breadth of what a JS developer should know.
Note: If you enjoy this article, please give it a clap 👏 (or 50!) to help spread the word!
I will be continuously updating this list in a Github repository called JS Tips & Tidbits. Please star ⭐ and share if you want to follow along!
Understanding how JavaScript assigns to variables is foundational to writing bug-free JavaScript. If you don’t understand this, you could easily write code that unintentionally changes values.
JavaScript always assigns variables by value. But this part is very important: when the assigned value is one of JavaScript’s five primitive type (i.e., Boolean
, null
, undefined
, String
, and Number
) the actual value is assigned. However, when the assigned value is an Array
, Function
, or Object
a reference to the object in memory is assigned.
Example time! In the following snippet, var2
is set as equal to var1
. Since var1
is a primitive type (String
), var2
is set as equal to var1
’s String
value and can be thought of as completely distinct from var1
at this point. Accordingly, reassigning var2
has not effect on var1.
let var1 = 'My string';
let var2 = var1;
var2 = 'My new string';
console.log(var1);
// 'My string'
console.log(var2);
// 'My new string'
Let’s compare this with object assignment.
let var1 = { name: 'Jim' }
let var2 = var1;
var2.name = 'John';
console.log(var1);
// { name: 'John' }
console.log(var2);
// { name: 'John' }
One might see how this could cause problems if you expected behavior like primitive assignment! This can get especially ugly if you create a function that unintentionally mutates an object.
Closure is an important JavaScript pattern to give private access to a variable. In this example, createGreeter
returns an anonymous function that has access to the supplied greeting
, “Hello.” For all future uses, sayHello
will have access to this greeting!
function createGreeter(greeting) {
return function(name) {
console.log(greeting + ', ' + name);
}
}
const sayHello = createGreeter('Hello');
sayHello('Joe');
// Hello, Joe
In a more real-world scenario, you could envision an initial function apiConnect(apiKey)
that returns some methods that would use the API key. In this case, the apiKey
would just need to be provided once and never again.
function apiConnect(apiKey) {
function get(route) {
return fetch(`${route}?key=${apiKey}`);
}
function post(route, params) {
return fetch(route, {
method: 'POST',
body: JSON.stringify(params),
headers: {
'Authorization': `Bearer ${apiKey}`
}
})
}
return { get, post }
}
const api = apiConnect('my-secret-key');
// No need to include the apiKey anymore
api.get('http://www.example.com/get-endpoint');
api.post('http://www.example.com/post-endpoint', { name: 'Joe' });
Don’t be thrown off by JavaScript parameter destructuring! It’s a common way to cleanly extract properties from objects.
const obj = {
name: 'Joe',
food: 'cake'
}
const { name, food } = obj;
console.log(name, food);
// 'Joe' 'cake'
If you want to extract properties under a different name, you can specify them using the following format.
const obj = {
name: 'Joe',
food: 'cake'
}
const { name: myName, food: myFood } = obj;
console.log(myName, myFood);
// 'Joe' 'cake'
In the following example, destructuring is used to cleanly pass the person
object to the introduce
function. In other words, destructuring can be (and often is) used directly for extracting parameters passed to a function. If you’re familiar with React, you probably have seen this before!
const person = {
name: 'Eddie',
age: 24
}
function introduce({ name, age }) {
console.log(`I'm ${name} and I'm ${age} years old!`);
}
console.log(introduce(person));
// "I'm Eddie and I'm 24 years old!"
A JavaScript concept that can throw people off but is relatively simple is the spread operator! In the following case, Math.max
can’t be applied to the arr
array because it doesn’t take an array as an argument, it takes the individual elements as arguments. The spread operator ...
is used to pull the individual elements out the array.
const arr = [4, 6, -1, 3, 10, 4];
const max = Math.max(...arr);
console.log(max);
// 10
Let’s talk about JavaScript rest syntax. You can use it to put any number of arguments passed to a function into an array!
function myFunc(...args) {
console.log(args[0] + args[1]);
}
myFunc(1, 2, 3, 4);
// 3
JavaScript array methods can often provide you incredible, elegant ways to perform the data transformation you need. As a contributor to StackOverflow, I frequently see questions regarding how to manipulate an array of objects in one way or another. This tends to be the perfect use case for array methods.
I will cover a number of different array methods here, organized by similar methods that sometimes get conflated. This list is in no way comprehensive: I encourage you to review and practice all of them discussed on MDN (my favorite JavaScript reference).
map, filter, reduce There is some confusion around the JavaScript array methods map
, filter
, reduce
. These are helpful methods for transforming an array or returning an aggregate value.
const arr = [1, 2, 3, 4, 5, 6];
const mapped = arr.map(el => el + 20);
console.log(mapped);
// [21, 22, 23, 24, 25, 26]
const arr = [1, 2, 3, 4, 5, 6];
const filtered = arr.filter(el => el === 2 || el === 4);
console.log(filtered);
// [2, 4]
const arr = [1, 2, 3, 4, 5, 6];
const reduced = arr.reduce((total, current) => total + current);
console.log(reduced);
// 21
find, findIndex, indexOf The array methods find
, findIndex
, and indexOf
can often be conflated. Use them as follows.
const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
const found = arr.find(el => el > 5);
console.log(found);
// 6
Again, note that while everything after 5
meets the criteria, only the first matching element is returned. This is actually super helpful in situations where you would normally break
a for loop when you find a match!
find
, but rather than returning the first matching element it returns the index of the first matching element. Take the following example, which uses names instead of numbers for clarity.const arr = ['Nick', 'Frank', 'Joe', 'Frank'];
const foundIndex = arr.findIndex(el => el === 'Frank');
console.log(foundIndex);
// 1
findIndex
, but instead of taking a function as an argument it takes a simple value. You can use this when you have simpler logic and don’t need to use a function to check whether there is a match.const arr = ['Nick', 'Frank', 'Joe', 'Frank'];
const foundIndex = arr.indexOf('Frank');
console.log(foundIndex);
// 1
push, pop, shift, unshift There are a lot of great array method to help add or remove elements from arrays in a targeted fashion.
let arr = [1, 2, 3, 4];
const pushed = arr.push(5);
console.log(arr);
// [1, 2, 3, 4, 5]
console.log(pushed);
// 5
let arr = [1, 2, 3, 4];
const popped = arr.pop();
console.log(arr);
// [1, 2, 3]
console.log(popped);
// 4
let arr = [1, 2, 3, 4];
const shifted = arr.shift();
console.log(arr);
// [2, 3, 4]
console.log(shifted);
// 1
let arr = [1, 2, 3, 4];
const unshifted = arr.unshift(5, 6, 7);
console.log(arr);
// [5, 6, 7, 1, 2, 3, 4]
console.log(unshifted);
// 7
splice, slice These methods either modify or return subsets of arrays.
The following code sample can be read as: at position 1
of the array, remove 0
elements and insert b
.
let arr = ['a', 'c', 'd', 'e'];
arr.splice(1, 0, 'b')
let arr = ['a', 'b', 'c', 'd', 'e'];
const sliced = arr.slice(2, 4);
console.log(sliced);
// ['c', 'd']
console.log(arr);
// ['a', 'b', 'c', 'd', 'e']
sort
let arr = [1, 7, 3, -1, 5, 7, 2];
const sorter = (firstEl, secondEl) => firstEl - secondEl;
arr.sort(sorter);
console.log(arr);
// [-1, 1, 2, 3, 5, 7, 7]
Phew, did you catch all of that? Neither did I. In fact, I had to reference the MDN docs a lot while writing this — and that’s okay! Just knowing what kind of methods are out there with get you 95% of the way there.
Don’t fear the *
. The generator function specifies what value
is yielded next time next()
is called. Can either have a finite number of yields, after which next()
returns an undefined value
, or an infinite number of values using a loop.
function* greeter() {
yield 'Hi';
yield 'How are you?';
yield 'Bye';
}
const greet = greeter();
console.log(greet.next().value);
// 'Hi'
console.log(greet.next().value);
// 'How are you?'
console.log(greet.next().value);
// 'Bye'
console.log(greet.next().value);
// undefined
And using a generator for infinite values:
function* idCreator() {
let i = 0;
while (true)
yield i++;
}
const ids = idCreator();
console.log(ids.next().value);
// 0
console.log(ids.next().value);
// 1
console.log(ids.next().value);
// 2
// etc...
===
) vs. Equality Operator (==
)Be sure to know the difference between the identify operator (===
) and equality operator (==
) in JavaScript! The ==
operator will do type conversion prior to comparing values whereas the ===
operator will not do any type conversion before comparing.
console.log(0 == '0');
// true
console.log(0 === '0');
// false
A mistake I see JavaScript newcomers make is directly comparing objects. Variables are pointing to references to the objects in memory, not the objects themselves! One method to actually compare them is converting the objects to JSON strings. This has a drawback though: object property order is not guaranteed! A safer way to compare objects is to pull in a library that specializes in deep object comparison (e.g., lodash’s isEqual).
The following objects appear equal but they are in fact pointing to different references.
const joe1 = { name: 'Joe' };
const joe2 = { name: 'Joe' };
console.log(joe1 === joe2);
// false
Conversely, the following evaluates as true because one object is set equal to the other object and are therefore pointing to the same reference (there is only one object in memory).
const joe1 = { name: 'Joe' };
const joe2 = joe1;
console.log(joe1 === joe2);
// true
Make sure to review the Value vs. Reference section above to fully understand the ramifications of setting a variable equal to another variable that’s pointing to a reference to an object in memory!
Far too many people are intimidated by JavaScript callback functions! They are simple, take this example. The console.log
function is being passed as a callback to myFunc
. It gets executed when setTimeout
completes. That’s all there is to it!
function myFunc(text, callback) {
setTimeout(function() {
callback(text);
}, 2000);
}
myFunc('Hello world!', console.log);
// 'Hello world!'
Once you understand JavaScript callbacks you’ll soon find yourself in nested “callback hell.” This is where Promises help! Wrap your async logic in a Promise and resolve
on success or reject
on fail. Use “then” to handle success and catch
to handle failure.
const myPromise = new Promise(function(res, rej) {
setTimeout(function(){
if (Math.random() < 0.9) {
return res('Hooray!');
}
return rej('Oh no!');
}, 1000);
});
myPromise
.then(function(data) {
console.log('Success: ' + data);
})
.catch(function(err) {
console.log('Error: ' + err);
});
// If Math.random() returns less than 0.9 the following is logged:
// "Success: Hooray!"
// If Math.random() returns 0.9 or greater the following is logged:
// "Error: On no!"
Once you get the hang of JavaScript promises, you might like async await
, which is just “syntactic sugar” on top of promises. In the following example we create an async
function and within that we await
the greeter
promise.
const greeter = new Promise((res, rej) => {
setTimeout(() => res('Hello world!'), 2000);
})
async function myFunc() {
const greeting = await greeter;
console.log(greeting);
}
myFunc();
// 'Hello world!'
If you didn’t know any of these 12 concepts, you likely have grown at least a little in your knowledge of JavaScript! And if you knew them all, then hopefully this was a chance to practice and grow your knowledge. What other concepts do you think are critical? Let me know in the comments.
30s ad
☞ How To Publish Your Vue.js Component On NPM - Vue.js Developers
☞ Build a PWA With Vue & Ionic 4
☞ Top 5 Uses for the Spread Operator in JavaScript
☞ Set Up a Vue.js Development Environment
#javascript