Justyn  Ortiz

Justyn Ortiz

1606722540

Type checking in JavaScript: Typeof and Instanceof Operators

Learn how to perform type checking in JavaScript using typeof and instanceof operators.

JavaScript is a loosely-typed language, so there is no restriction on the variable’s type.

For example, if you’ve created a variable with a string type, later you can assign to the same variable a number:

let message = 'Hello'; // assign a string

message = 14; // assign a number

Such dynamism gives you flexibility and simplifies variables declaration.

On the other side, you can never be sure that a variable contains a value of a certain type. For example, the following function greet(who) expects a string argument, however, you can invoke the function with any type of argument:

function greet(who) {
  return `Hello, ${who}!`
}

greet('World'); // => 'Hello, World!'
// You can use any type as argument
greet(true);    // => 'Hello, true!'
greet([1]);     // => 'Hello, 1!'

That’s why, sometimes, you need to check the variable’s type in JavaScript — using typeof operator, as well as instanceof to check instance types.

Let’s see in more detail how to use typeof and instanceof operators in JavaScript.

  • typeof operator
    • typeof null
    • typeof and not defined variables
  • instanceof operator
    • instanceof and the parent class

#javascript #web-development #testing #programming

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Type checking in JavaScript: Typeof and Instanceof Operators
Coy  Roberts

Coy Roberts

1599716220

Javascript Typeof Example | typeof Operator Tutorial

Javascript typeof is an inbuilt operator that returns the string indicating a type of the unevaluated operand. JavaScript typeof operator returns a data type of its operand in the form of the string. The operand can be any object, function or variable. You can use a JavaScript typeof operator to find the type of the JavaScript variable. The typeof operator returns the type of the variable or an expression.

Javascript Typeof Example

Let’s see the syntax of the typeof operator.

typeof operand

The operand parameter is an expression representing the object or primitive whose type is to be returned. See the following example.

// app.js

console.log(typeof 'appdividend');

#javascript #javascript typeof #typeof operator #typeof

Santosh J

1622036598

JavaScript compound assignment operators

JavaScript is unarguablly one of the most common things you’ll learn when you start programming for the web. Here’s a small post on JavaScript compound assignment operators and how we use them.

The compound assignment operators consist of a binary operator and the simple assignment operator.

The binary operators, work with two operands. For example a+b where + is the operator and the a, b are operands. Simple assignment operator is used to assign values to a variable(s).

It’s quite common to modify values stored in variables. To make this process a little quicker, we use compound assignment operators.

They are:

  • +=
  • -+
  • *=
  • /=

You can also check my video tutorial compound assignment operators.

Let’s consider an example. Suppose price = 5 and we want to add ten more to it.

var price = 5;
price = price + 10;

We added ten to price. Look at the repetitive price variable. We could easily use a compound += to reduce this. We do this instead.

price += 5;

Awesome. Isn’t it? What’s the value of price now? Practice and comment below. If you don’t know how to practice check these lessons.

Lets bring down the price by 5 again and display it.
We use console.log command to display what is stored in the variable. It is very help for debugging.
Debugging let’s you find errors or bugs in your code. More on this later.

price -= 5;
console.log(price);

Lets multiply price and show it.

price *=5;
console.log(price);

and finally we will divide it.

price /=5;
console.log(price);

If you have any doubts, comment below.

#javascript #javascript compound assignment operators #javascript binary operators #javascript simple assignment operator #doers javascript

Javascript Instanceof Operator Example | Instanceof in Javascript

Javascript instanceof is an inbuilt operator that tests whether a prototype property of the constructor appears anywhere in the prototype chain of an object. The instanceof operator examines the presence of constructor.type in the object‘s prototype chain. The instanceof operator returns the boolean value that indicates if an object is an instance of the particular class.

Javascript Instanceof Operator

The instanceof operator is used to check the type of the object at a run time. The instanceof operator tests a presence of constructor.prototype in the object‘s prototype chain.

It checks the current object and returns true if the object is of the specified object type.

The Left Hand Side (LHS) operand is an actual object being tested to the Right Hand Side (RHS) operand, which is an actual constructor of a class.

#javascript #javascript instanceof #js

Javascript Instanceof Operator Example | Instanceof in Javascript

Javascript instanceof is an inbuilt operator that tests whether a prototype property of the constructor appears anywhere in the prototype chain of an object. The instanceof operator examines the presence of constructor.type in the object‘s prototype chain. The instanceof operator returns the boolean value that indicates if an object is an instance of the particular class.

Javascript Instanceof Operator

The instanceof operator is used to check the type of the object at a run time. The instanceof operator tests a presence of constructor.prototype in the object‘s prototype chain.

It checks the current object and returns true if the object is of the specified object type.

The Left Hand Side (LHS) operand is an actual object being tested to the Right Hand Side (RHS) operand, which is an actual constructor of a class.

See the following syntax.

object instanceof constructor

The object is to be tested, and the constructor is the function to test against.

#javascript #javascript instanceof #js

Nigel  Uys

Nigel Uys

1669121709

Check The Type Of Your JavaScript Object using The instanceof Operator

Check the type of your JavaScript object using the instanceof operator.

The JavaScript instanceof operator is used to find out the type of objects or values you have in your code.

The instanceof syntax is as follows:

object instanceof Object;

// returns a boolean: true or false

The instanceof will test if the Object constructor is present in the object’s prototype chain.

The example code below shows how you can use the instanceof method to test the Person object instance:

class Person {}

let person = new Person();

console.log(person instanceof Person); // true
console.log(person instanceof Object); // true
console.log(person instanceof Boolean); // false

Because the person variable’s value is an instance of both Person and Object classes, the instanceof operator returns true.

But the person object is not an instance of the Boolean class, so the operator returns false.

The instanceof operator also works when you create an object from a function as shown below:

function Dog() {}

let dog = new Dog();

console.log(dog instanceof Dog); // true

As you can see, the dog variable is an instance of Dog, so the instanceof operator returns true.

JavaScript instanceof vs typeof

JavaScript also has the typeof operator that can be used to check the type of values you define in your code.

The typeof operator returns a string representing the type of the value. It is commonly used for built-in JavaScript types as shown below:

console.log(typeof "Hello"); // "string"
console.log(typeof 123); // "number"

The instanceof and typeof operators work in different ways.

The instanceof returns either true or false while the typeof returns the actual type name of the value.

In general, the instanceof is used for testing advanced and custom types, while typeof is used for common JavaScript types.

The code below shows the difference between the two operators:

class Person {}

let person = new Person();

console.log(person instanceof Person); // true
console.log(typeof person); // "object"

console.log("Hello" instanceof String); // false
console.log(typeof "Hello"); // "string"
console.log(typeof "Hello" == "string"); // true

console.log(true instanceof Boolean); // false
console.log(typeof true); // "boolean"

console.log(true instanceof "boolean"); // ERROR

Now you’ve learned how the instanceof operator works in JavaScript, as well as the difference between the instanceof and typeof operators.

I hope this tutorial has been useful for you. 🙏

Original article source at: https://sebhastian.com/

#javascript #object #type