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JavaScript Function Definitions


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

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

function functionName(parameters) {
  // code to be executed
}


Declared functions are not executed immediately. They are "saved for later use",
and will be executed later, when they are invoked (called upon).

Example

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

Try it Yourself »


Semicolons are used to separate executable JavaScript statements.
Since a function declaration 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 an expression.
A function expression can be stored in a variable:

Example

const x = function (a, b) {return a * b};

Try it Yourself »

After a function expression has been stored in a variable, the variable can
be used as a function:

Example

const x = function (a, b) {return a * b};
let z = x(4, 3);
Try it Yourself »

The function above is actually an anonymous function (a function without a
name).
Functions stored in variables do not need function names. They are always
invoked (called) using the variable name.

The function above ends with a semicolon because it is a part of an executable statement.








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

Example

const myFunction = new Function("a", "b", "return a * b");
let x = myFunction(4, 3);
Try it Yourself »

You actually don't have to use the function constructor. The example above is the same as writing:

Example

const myFunction = function (a, b) {return a * b};
let x = myFunction(4, 3);
Try it Yourself »


Most of the time, you can avoid using the new keyword in JavaScript.


Function Hoisting
Earlier in this tutorial, you learned about "hoisting" (JavaScript Hoisting).
Hoisting is JavaScript's default behavior of moving declarations to the top of the
current scope.
Hoisting applies to variable declarations and to function declarations.
Because of this, 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 expression is invoked (started) automatically, without being called.
Function expressions 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 indicate that it is a function expression:

Example

(function () {
  let x = "Hello!!";  // I will invoke myself
})();

Try it Yourself »

The function above is actually an 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;}
let x = myFunction(4, 3);
Try it Yourself »

JavaScript functions can be used in expressions:

Example

function myFunction(a, b) {  return a * b;}
let x = myFunction(4, 3) * 2;
Try it Yourself »


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

Example

function myFunction(a, b) {  return arguments.length;}
Try it Yourself »

The toString() method returns the function as a string:

Example

function myFunction(a, b) {  return a * b;}let text = myFunction.toString();
Try it Yourself »


A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.


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

Example

// ES5
var x = function(x, y) {
 
return x * y;
}
// ES6
const x = (x, y) => x * y;

Try it Yourself »

Arrow functions do not have their own this.
They are not well suited for defining object methods.
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 constant value.
You can only omit the return keyword and the curly brackets if the function is a single statement.
Because of this, it might be a good habit to always keep them:

Example

const x = (x, y) => { return x * y };

Try it Yourself »


Arrow functions are not supported in IE11 or earlier.













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Reference: https://www.w3schools.com/js/js_function_definition.asp