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).

See an example

function sampleFunc(a, b) {
    return a + b
}

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

Function names can contain letters, digits, underscores, and dollar signs.

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

NOTE A Function is much the same as a Procedure or a Subroutine, in other programming languages.

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

• When an event occurs
• When it is invoked from code
• Automatically (self-invoked)

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

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

var multiply = multiplyFunc(4, 5);  // Function is called

function multiplyFunc(a, b){
    return a * b;                  // return products of a,b
}

// Output
20

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.

function toCelsius(fahrenheit) {
  return (5/9) * (fahrenheit-32);
}

// Output
25

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

// Instead of using this
var degree = toCelsius(77);

// You can use function directly as variable
var roomTemp = "The temperate is " + toCelsius(77) + "Celsius"

A function can be declared or created in couple of ways:

• Declaration function
• Expression Function
• Anonymouse Function
• Arrow Function

Function can be declared without parameter and even without return.

function Cube() {
    let num = 3
    let cube = num * num * num;
    console.log(cube)
}

// Invoking function
Cube()                  // 9

In a function we can pass different data types(number, string, boolean, object, function) as a parameter.

function AnySense(param1) {
    return param1
}

function areaOfCircle(radius) {
    let area = Math.PI * r * r
    return area
}

Sometimes we do not know how many arguments the user going to pass. Therefore, we should know how to write a function which can take unlimited number of arguments.

We will be using arrow function to handle the unlimited number of scope.

const sumAllNums = (...args) => {
    let sum = 0;
    for (const element in args) {
        sum += element
    }
    return sum
}

sumAllNums(1,2,3,4)
//Output
10

The above function is also know as arrow function.

Anonymous function or without name

const noName = function() {
    return whatever
}

Expression functions are anonymous functions. After we create a function without a name and we assign it to a variable.

// Function expression
const square = function(n) {
  return n * n
}
square(2)

Self invoking functions are anonymous functions which do not need to be called to return a value.

(function(n) {
    return n * n
})(2)

let cubeNum = (function(n) {
    return n * n * n;
})(10)

//Output
1000

Arrow function is an alternative to write a function, however function declaration and arrow function have some minor differences.

const square = n => {
  return n * n
}

square(3)
// Output
9

One more example

onst changeToUpperCase = arr => {
  const newArr = []
  for (const element of arr) {
    newArr.push(element.toUpperCase())
  }
  return newArr
}

const countries = ['Finland', 'Sweden', 'Norway', 'Denmark', 'Iceland']
changeToUpperCase(countries)
// ["FINLAND", "SWEDEN", "NORWAY", "DENMARK", "ICELAND"]

🍻🎉💊 Congrats for completing Day 2 🍻🎉💊

if you are preparing for interview have a look at interview questions of Functions