JavaScript Function Definitions | javascript function

JavaScript functions are defined with the function keyword.
You can use a function declaration or you can use a function expression.

Function Declarations

Earlier in this tutorial, you learned that functions are declared with following syntax:

function functionName(parameters) {
  // code to be executed}
Declared functions are not executed immediately. They are "saved for later uses", and will be executed later when they invoked (called upon).

Example

function myFunction(a, b) {
  return a * b;
}
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Semicolons are used to separate executable JavaScript statements.
Since a function declarations is not an executable statement, it is not common to end it with a semicolon.

Function Expressions

A JavaScript function can also be defined using a expression.
A function expressions can be stored in a variable:

Example

var x = function (b, c) {return b * c};
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After a function expression has been store in a variable and the variable can be used as a function:

Example

var x = function (b, c) {return b * c};
var z = x(43);
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The function above is actually a anonymous function (a function without  name).
Functions stored in a variables do not need function names. They are alway invoked (called) using the variable name.
The function above ends with a semicolon because it is a part of an executable statements.


The Function() Constructor

As you have seen in the previous example, JavaScript functions are defined with the function keyword.
Functions can also be define with a built-in JavaScript function constructor called Function().

Example

var newFunction = new Function("a""b""return a * b");

var x = newFunction(43);
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actually you don't have to use the function constructor. The example above is the same as writing:

Example

var myFunction = function (a, b) {return a * b};

var x = myFunction(43);
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Most of the time, you can avoid using the keywordnew in JavaScript.

Function Hoisting

Earlier in this tutorials, you learned about "hoisting" (JavaScript Hoisting).
Hoisting is JavaScript default behaviors of moving declarations to the top of the current scope.
Hoisting applies to variable declaration and to function declarations.
Because JavaScript functions can be called before they are declared:

myFunction(5);

function myFunction(y) {
  return y * y;
}
Functions defined using an expression are not hoisted.

Self-Invoking Functions

Function expressions can be made "self-invoking".
A self-invoking expressions is invoked (started) automatically, without being called.
Function expressions be will execute automatically if the expression is followed by ().
You cannot self-invoke a function declaration.
You have to add parentheses around the function to indicates that it is a function expression:

Example

(function () {
  var x = "Hello!!";  // I will invoke myself})();
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The function above is actually a anonymous self-invoking function (function without name).

Functions Can Be Used as Values

JavaScript functions can be used as values:

Example

function myFunction(a, b) {
  return a * b;
}

var x = myFunction(43);
Try it Yourself »
JavaScript functions can be used in expressions:

Example

function myFunction(a, b) {
  return a * b;
}

var x = myFunction(43) * 2;
Try it Yourself »

Functions are Objects

The typeof operator in JavaScript return "function" for functions.
But, JavaScript function can best be described as objects.
JavaScript functions have both properties and methods.
The arguments.length property return the number of arguments received when the function was invoked:

Example

function myFunction(a, b) {
  return arguments.length;
}
Try it Yourself »
The toString() method return the function as a string:

Example

function myFunction(a, b) {
  return a * b;
}

var txt = myFunction.toString();
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A function defined as the property of a object, is called a method to the object.
A function designed to create a new objects, is called an object constructor.

Arrow Functions

Arrow functions allows a short syntax for writing functions expressions.
You don't need the keywordfunction, the return keyword, and the curly brackets.

Example

// ES5var x = function(x, y) {
  return x * y;
}

// ES6const x = (x, y) => x * y;
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Arrow functions do not have their own this. They are not well suited for defining a object method.
Arrow functions are not hoisted. They must be defined before they are used.
Using const is safer than using var, because a function expression is always a constant value.
You can only omit the keywordreturn and the curly brackets if the function is a single statement. Because of this, it might be good habit to always keep them:

Example

const x = (x, y) => { return x * y };
Try it Yourself »
Arrow function are not supported in IE11 or earlier.

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