Array & Function

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