For the month of December I have challenged myself to write 24 articles about JS up until Xmas.

The second installment in this series is around Mutable and Immutable array methods.

What is a mutable method?

• A mutable method is a method which changes the state of the array it is called on. eg. splice, pop, push, unshift, shift, reverse

What is an immutable method?

• An immutable method is a method which does not change the state of the array it is called on and sometimes returns a new array instead. eg. concat, map, slice, filter

Examples of array mutation methods

pop:

let arr = [1,2,3,4];
console.log(arr); // [1,2,3,4]
arr.pop();
console.log(arr); // [1,2,3]

push:

// reset the array
let arr = [1,2,3,4];
arr.push(5);
console.log(arr); // [1,2,3,4,5]

reverse:

 // reset the array
let arr = [1,2,3,4];
arr.reverse();
console.log(arr); // [4,3,2,1]

unshift:

// reset the array
let arr = [1,2,3,4]; 
arr.unshift(1);
console.log(arr); // [1,1,2,3,4]

• The value of the original array has changed in every instance.

Examples of array immutable methods

filter:

let arr = [1,2,3,4];
let newArray = arr.filter(el => el > 2);
console.log(arr); // [1,2,3,4]
console.log(newArray); // [3,4]

map:

let arr = arr.map(el => el * 2);
console.log(arr); // [1,2,3,4]
console.log(newArray); // [2,4,6,8]

concat:

let newArray = arr.concat([5,6,7]);
console.log(arr); // [1,2,3,4]
console.log(newArray); // [1,2,3,4,5,6,7]

slice:

let newArray = arr.slice(2)
console.log(arr); // [1,2,3,4]
console.log(newArray); // [3,4,]

• The original array stays the same when the immutable methods are called on it.

Pure functions

• A pure function is a function which as well as returning the same result every time it is executed with the same inputs also does not have any other side affects.
• Using immutable array methods in functions contributes to this as they do not produce the side effect of changing the array.

< JS Advent #1