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02_JavaScript_Array_&_Function.md

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JavaScript Arrays and Functions



JavaScript Arrays


Array

In JavaScript, arrays consist only of values


  • Array literal

    var a = [1,2,3]

  • Array constructor function

    var b = new Array(1,2,3)


Array Iteration

  • for

    var a = [1,2,3]

for (var i=0 ; i <a.length; i++){
    console.log(i, a[i])
}

  • for .. of

    : Outputs element values one by one

    for (var elem of a){
    console.log(elem)
}

  • forEach

    : Takes a function itself as an argument

    a.forEach( function(elem, idx) {
    console.log(idx, elem)
})

  • for .. in

    • Not only accesses array elements but properties can also be output
      • Since arrays in JavaScript are also objects, property settings are possible, but they become properties of the Object, not the list
    var a = [1,2,3]

a.name = 'myarr'

for (var i in a){
    console.log(i, a[i])
}

//0 1
//1 2
//2 3
//name myarr   => iterates through properties as well
    • Be careful when using the for ... in form!
      • for ...in iterates through all properties of the Object itself


Array methods


  • sort
    • If there is no comparison function (argument), it sorts based on strings
      • If you don't want that, pass a comparison function as an argument
    • If there is a comparison function, it sorts based on the return value being less than 0

  • String-related

    • join
      • array.join('haha')

  • toString

  • Array merging

    • concat

      : Merges two arrays

  • Element insertion/deletion

    • push
    • pop
    • unshift
      • Insert at the left end
    • shift
      • Remove from the left end

  • Index search

    • indexOf

  • Array manipulation

    • splice(start[, deleteCount[, item1[, item2[, ...]]]])

      • Modifies the original array itself

      • Can also modify/delete elements

        var a = [1,2,3]
a.splice(1,2, "first", "second") // Delete two from position 1 + append "first", "second"
// [1, "first", "second"]

var a = [1,2,3]
a.splice(1,2, "first") // Delete two from position 1 + append "first"
// [1, "first"]

  • Array slicing

    • slice

      : Returns the result



JavaScript Functions


Function Declaration


1. Function Declaration Statement

function sum(a,b) {
    return a+b;
}
sum(1,2) //3

2. Function Expression

// Assign an anonymous function to a variable
var sub = function(a,b) {
    return a - b;
}
sub(1,2) //-1

3. Immediately Invoked Function Expression (IIFE)

(function(a,b){return a*b})(1,2) //2

4. Arrow Function (ES6)

var sum = (a,b) => a+b  // Single line a+b so curly braces {} are omitted
sum(3,4) //7

var area = (r) => {
    const PI = 3.14;
    return r*r*PI;
}
area(1) //3.14


Function Arguments

  • In JavaScript, there are no restrictions on passing parameters to functions

  • The arguments object contains all information about the parameters passed

    function foo(a) {
    console.log(arguments); // Print arguments to check
    return a
}

    • If no arguments are passed, it shows undefined

      • undefined

        : The value JavaScript assigns when initializing a variable