Explain Call, Apply and Bind in JavaScript.

Call, Apply, and Bind in JavaScript

In JavaScript, call(), apply(), and bind() are methods that allow you to control the value of this when calling a function. They are especially useful when working with objects and borrowing methods.

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1. call() Method

The call() method invokes a function immediately with a specific value for this, and you can pass arguments individually.

Syntax:

javascriptCopyEditfunctionName.call(thisArg, arg1, arg2, ...)

Example:

javascriptCopyEditconst person = {
    name: 'Alice',
    greet: function (city) {
        console.log(`Hello, my name is ${this.name} and I live in ${city}`);
    }
};

const person2 = { name: 'Bob' };

person.greet.call(person2, 'New York');
// Output: Hello, my name is Bob and I live in New York

Key Points:

• call() calls the function immediately.
• thisArg is the value of this inside the function.
• Arguments are passed individually.

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2. apply() Method

The apply() method is similar to call(), but it takes arguments as an array instead of individual values.

Syntax:

javascriptCopyEditfunctionName.apply(thisArg, [arg1, arg2, ...])

Example:

javascriptCopyEditconst person = {
    name: 'Alice',
    greet: function (city, country) {
        console.log(`Hello, my name is ${this.name} and I live in ${city}, ${country}`);
    }
};

const person2 = { name: 'Bob' };

person.greet.apply(person2, ['Los Angeles', 'USA']);
// Output: Hello, my name is Bob and I live in Los Angeles, USA

Key Points:

• apply() calls the function immediately.
• Arguments are passed as an array.
• Useful when you don’t know the number of arguments in advance (e.g., Math.max.apply(null, [1, 2, 3])).

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3. bind() Method

The bind() method returns a new function with a specific value for this, but does not call the function immediately.

Syntax:

javascriptCopyEditconst boundFunction = functionName.bind(thisArg, arg1, arg2, ...)

Example:

javascriptCopyEditconst person = {
    name: 'Alice',
    greet: function (city) {
        console.log(`Hello, my name is ${this.name} and I live in ${city}`);
    }
};

const person2 = { name: 'Bob' };

const boundGreet = person.greet.bind(person2, 'Chicago');
boundGreet(); // Output: Hello, my name is Bob and I live in Chicago

Key Points:

• bind() does not call the function immediately.
• Returns a new function with this permanently set to thisArg.
• Useful when passing functions as callbacks or preserving this context in asynchronous operations.

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Key Differences Summary:

Method	Calls the Function Immediately?	Arguments Passed Individually or as an Array?	Returns a New Function?
call()	Yes	Individually	No
apply()	Yes	As an array	No
bind()	No	Individually	Yes

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When to Use Which?

Use Case	Recommended Method
Call a function immediately with a specific this	call() or apply()
Pass arguments as an array	apply()
Create a new function with this bound for later execution	bind()
Event handlers, callbacks that need a fixed this	bind()

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Practical Example Combining All:

javascriptCopyEditconst person = {
    name: 'Alice',
    age: 25
};

function introduce(city, country) {
    console.log(`Hi, I'm ${this.name}, ${this.age} years old, from ${city}, ${country}`);
}

// call
introduce.call(person, 'New York', 'USA');

// apply
introduce.apply(person, ['London', 'UK']);

// bind
const boundIntroduce = introduce.bind(person, 'Paris', 'France');
boundIntroduce();

Output:

rustCopyEditHi, I'm Alice, 25 years old, from New York, USA
Hi, I'm Alice, 25 years old, from London, UK
Hi, I'm Alice, 25 years old, from Paris, France