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JavaScript Using Constants with Arrays and Objects

Posted by A X
A X
12,842 Points

Advantages/Disadvantages of Using Const vs. Var in an Array

Just finished watching the video where Andrew uses const to create variables in an array vs. the var keyword. I found the video confusing as I couldn't quite follow the const keyword's behavior, and the advantages/disadvantages of using const with an array vs. var. It seems to me since arrays frequently change that you should never or rarely use const, but that wasn't the impression I was left with at the end of this video. Can someone elaborate? Thanks!

4 Answers

Seth Kroger
Seth Kroger
56,413 Points
Seth Kroger
Seth Kroger
56,413 Points

Arrays are what's called a "reference variable" in that they refer to, or point to a value in memory instead of directly holding the value. This distinction doesn't often make much difference but here it does. With const the effect is only one level deep, so can't change which array it points to, but you are allowed to alter the array.

const array1 = [1,2,3,4,5]  // This creates the array in memory and array1 points to it.
const array2 = array1  // This doesn't copy the array, only the reference pointing to the array.

  // can change the underlying array
array2.pop()
  // <- 5
array1
  // <- Array [ 1, 2, 3, 4 ]  The 5 is gone because array1 and array2 are the same array

//But you can't change which array it points to:
array1 = [6,7,8,9]
  // <- TypeError: invalid assignment to const `array1'
ywang04
ywang04
6,762 Points
ywang04
ywang04
6,762 Points

@Seth Kroger We understand what you mentioned above. Actually running the following code with var keyword, it still get the same result.

var array1 = [1,2,3,4,5]  // This creates the array in memory and array1 points to it.
var array2 = array1  // This doesn't copy the array, only the reference pointing to the array.

  // can change the underlying array
array2.pop()
  // <- 5
array1
  // <- Array [ 1, 2, 3, 4 ]  The 5 is gone because array1 and array2 are the same array

Only difference is const array1 cannot be reassigned while var array1 can be reassigned.

I have the same question like nekilof. What kind of situation we need to use const keyword with the array or object literal and what the advantage of using const in real projects? Thanks in advance. :)

Ali Raza
Ali Raza
13,633 Points
Ali Raza
Ali Raza
13,633 Points

CONST vs VAR

Think of Const and Var as a placeholder, now the difference between them is Var is changeable e.g. If var name had a string "Ali" it can later be changed to "Raza" which is why it is called a "variable"; whereas Const stays constant so if you create a const name APR = 24.1 percent, it will stay the same throughout. That way you can't mess it up with something else when it was not intended. It is just to stay on the safer side.

JS doesn't have strict type variables but in some other languages you cannot change the type of a variable for example a string type variable will always hold a string value. You can check the MDN Reference Guide for more info.

jason chan
jason chan
31,009 Points
jason chan
jason chan
31,009 Points

const

const means the variable can't be changed // so your code will error out on the compiler if it changes later in the program

let

let is another way to define the variable but it can mutate (change)

const carTires = 4;
let engineHp = 5, accelerate = true;
if(accelerate) {
   engineHp * 5 // 25
}
carTires = 5; // this will error

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

var is es5 is to define a variable. Javascript is trying to be more classically typed language. Typed language is like java where you define all your datatypes.

We also have es2015 (new version of javascript) class here taught by Guil. So you can have stronger understanding of these new features in javascript.

https://teamtreehouse.com/library/introducing-es2015

If you take the java course here at teamtreehouse you will see what classically typed language is like.

https://teamtreehouse.com/tracks/java-web-development

I hope that answers your question. You'll understand once your get little more progress in your coding career.

Prabal Dutta
Prabal Dutta
304 Points
Prabal Dutta
Prabal Dutta
304 Points

ywang04 What kind of situation we need to use const keyword with the array or object literal and what the advantage of using const in real projects?

The straight answer is you don't use const in case of arrays and objects. CONST works on variables but not very useful in case of arrays and objects.