Javascript | Declaring and calling functions | Call | Bind | Apply

Functions are an essential part of programming in JavaScript, allowing developers to write reusable code blocks that can be called from anywhere in the program. In this article, we will discuss how to declare and call functions in JavaScript, and also cover call, bind, and apply with examples.

Declaring a Function: A function is declared using the function keyword, followed by the function name, and the function parameters in parentheses. The function body is enclosed in curly braces. Here is an example of a function that takes two parameters and returns their sum:

function addNumbers(num1, num2) {
  return num1 + num2;
}

Calling a Function: A function can be called by using the function name, followed by parentheses that contain the arguments if any. Here is an example of calling the addNumbers function from above:

var sum = addNumbers(5, 10);
console.log(sum); // 15

Call, Bind, and Apply: In addition to declaring and calling functions, JavaScript provides three methods – call, bind, and apply – that allow us to manipulate how functions are executed.

call: The call method is used to invoke a function with a specific this context, and allows us to pass in arguments as individual parameters. Here is an example of using the call method:

var person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
};

var fullName = person.fullName.call();
console.log(fullName); // "John Doe"

In the above example, we use the call method to call the fullName function with the person object as the this context.

bind: The bind method is used to create a new function with a specific this context, and returns the new function that can be called later. Here is an example of using the bind method:

var person = {
  firstName: "John",
  lastName: "Doe",
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
};

var getFullName = person.fullName.bind(person);
console.log(getFullName()); // "John Doe"

In the above example, we use the bind method to create a new function called getFullName that has person as its this context.

apply: The apply method is used to invoke a function with a specific this context, and allows us to pass in arguments as an array. Here is an example of using the apply method:

var numbers = [5, 10];

var sum = addNumbers.apply(null, numbers);
console.log(sum); // 15

In the above example, we use the apply method to call the addNumbers function with null as the this context, and pass in the numbers array as arguments.

In conclusion, functions are a crucial part of programming in JavaScript, and knowing how to declare and call them is essential for any developer. Additionally, understanding how to use the call, bind, and apply methods can help us to write more efficient and effective code.