Function

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

Function Declaration:-

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;

}

myFunction(2,5) // 10

Function Expression:-

A JavaScript function can also be defined using an expression. Example:-

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

let z = x(4, 3); // 12

● 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.

Function Constructor:-

● 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); // 12

You actually don't have to use the function constructor.

Function Hoisting:-

● Hoisting is JavaScript's default behaviour of moving declarations to the top of the current scope.

● Hoisting applies to variable declarations and to function declarations. Example:-

myFunction(5);

function myFunction(y) {

return y * y;

} // 25

Functions defined using an expression are not hoisted.

Self-invoking Function:-

● 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 called myself"; // I will invoke myself })();

// Hello! I called myself

The function above is actually an anonymous self-invoking function (function without name).