What is Function Expression in Javascript

Table of Contents

• Introduction
• Functions
• Functions in Javascript
• Function Expression
  • Anonymous Function Expression
• Named Function Expression

Introduction

To understand Function Expression lets explore functions in Javascript first.

Functions

In general, Functions are subprograms which may, or may not take parameters(input) and return a value.

Functions in Javascript

A javascript function does the same thing. It takes parameters as input and returns a value.

function addTwoNumbers(a, b) {
	return a+b;
}

Javascript function definition starts with the function keyword, followed by:

• name of the function
• parameters if any, wrapped with parenthesis and separated by a comma
• the function body - series of statements inside curly brackets { }.

function greetings(name) {
	alert('Hi' + name);
}

If you have experience in other programming languages, the above function definitions will look very familiar to you.

Function Expression

Functions are first-class objects in Javascript. For that reason, they can be assigned to a variable as an expression, passed as a parameter, and much more.

A function expression is defined by assigning a function definition to a javascript variable.

var addTwoNumbers = function (a,b) {
	return a + b;
}
addTwoNumbers(2, 3);

Anonymous Function Expression

In the previous example, the function assigned to variable "addTwoNumbers" has no name. Therefore, it is called an anonymous function.

var multiplyTwoNumbers = function (a,b) {
	return a * b;
}
multiplyTwoNumbers(2, 3);

Anonymous Function Expressions are used to create callback functions. We will save callback for another time.

Named Function Expression

To create a named function expression, assign a function with a name to a variable. In this case, the scope of the function name is the function itself.

var newSeries = function fibonacci(num) {
  if (num <= 1) return 1;

  return fibonacci(num - 1) + fibonacci(num - 2);
}

Named Function Expressions are useful for creating recursive functions.

Function Expression is quite useful in the real world. It is also used to create a coding pattern called IIFE(Immediately Invoked Function Expression). We will discuss the IIFE in the next article.

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