What is function () () in javascript?

A JavaScript function is a block of code designed to perform a particular task.

A JavaScript function is executed when "something" invokes it (calls it).

Example

// Function to compute the product of p1 and p2
function myFunction(p1, p2) {
  return p1 * p2;
}

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JavaScript Function Syntax

A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().

Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).

The parentheses may include parameter names separated by commas:
(parameter1, parameter2, ...)

The code to be executed, by the function, is placed inside curly brackets: {}

function name(parameter1, parameter2, parameter3) {
  // code to be executed
}

Function parameters are listed inside the parentheses () in the function definition.

Function arguments are the values received by the function when it is invoked.

Inside the function, the arguments (the parameters) behave as local variables.

Function Invocation

The code inside the function will execute when "something" invokes (calls) the function:

• When an event occurs (when a user clicks a button)
• When it is invoked (called) from JavaScript code
• Automatically (self invoked)

You will learn a lot more about function invocation later in this tutorial.

Function Return

When JavaScript reaches a return statement, the function will stop executing.

If the function was invoked from a statement, JavaScript will "return" to execute the code after the invoking statement.

Functions often compute a return value. The return value is "returned" back to the "caller":

Example

Calculate the product of two numbers, and return the result:

let x = myFunction(4, 3);   // Function is called, return value will end up in x

function myFunction(a, b) {
  return a * b;             // Function returns the product of a and b
}

The result in x will be:

12

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Why Functions?

You can reuse code: Define the code once, and use it many times.

You can use the same code many times with different arguments, to produce different results.

Example

Convert Fahrenheit to Celsius:

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);
}
document.getElementById("demo").innerHTML = toCelsius(77);

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The () Operator Invokes the Function

Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.

Accessing a function without () will return the function object instead of the function result.

Example

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);
}
document.getElementById("demo").innerHTML = toCelsius;

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Functions Used as Variable Values

Functions can be used the same way as you use variables, in all types of formulas, assignments, and calculations.

Example

Instead of using a variable to store the return value of a function:

let x = toCelsius(77);
let text = "The temperature is " + x + " Celsius";

You can use the function directly, as a variable value:

let text = "The temperature is " + toCelsius(77) + " Celsius";

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You will learn a lot more about functions later in this tutorial.

Local Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables can only be accessed from within the function.

Example

// code here can NOT use carName

function myFunction() {
  let carName = "Volvo";
  // code here CAN use carName
}

// code here can NOT use carName

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Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Local variables are created when a function starts, and deleted when the function is completed.

Test Yourself With Exercises

Exercise:

Execute the function named myFunction.

function myFunction() {
  alert("Hello World!");
}
;

Start the Exercise

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).

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:

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

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);

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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);

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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
})();

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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);

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JavaScript functions can be used in expressions:

Example

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

let x = myFunction(4, 3) * 2;

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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:

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

Example

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

let text = myFunction.toString();

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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;

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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:

Arrow functions are not supported in IE11 or earlier.

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