CODEWITHSUNDEEP
javascript
Nghi Nguyen
Nghi Nguyen

Reputation: 950

The modifying in nested array

I created an nested array with empty array for each item. I want to input the data to each index with push method. But instead of inserting to each item the data is filled to all item. Could someone help me explain this behavior? Thanks

let input = new Array(4).fill([]);
let data = [[0,1],[3,2],[2,1]];
for(let i = 0; i < data.length; i++){
   const key = data[i][1]
   const value = data[i][0]
   input[key].push(value)
}
console.log(input)

Upvotes: 1

Views: 52

Answers (4)

juniorsenior
juniorsenior

Reputation: 66

Arrays that was filled inside input variable is the same and when you change one of this array you change all linked arrays also. You can read more about it here: https://javascript.info/object-copy

let input = new Array(4).fill([]);
let a = []
let b = []
let c = a

console.log(input[0] === input[1]) ---> true
console.log(a === b) ---> false
console.log(a === c) ---> true

Upvotes: 0

everett1992
everett1992

Reputation: 2661

You've filled the array with the same empty array

Think of it like this

const a = []
const b = new Array(4).fill(a)

a is inserted into the array four times and any index of b is a reference to a.

Upvotes: 1

Berk Kurkcuoglu
Berk Kurkcuoglu

Reputation: 1523

It is related to how Array fill behaves. When you pass an object (an empty array in this case) as the parameter, all of your elements have the same reference.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill#value

Upvotes: 1

Karthikeyan
Karthikeyan

Reputation: 414

Array is an object.. you are filling the array with same object and pushing to same object reference

let input = new Array();
let data = [[0,1],[3,2],[2,1]];
for(let i = 0; i < data.length; i++){
   input[key] = input[key] || [];
   const key = data[i][1]
   const value = data[i][0]
   input[key].push(value)
}
console.log(input)

Upvotes: 1

Related Questions