snapshot of a variable javascript

Question

I'm trying to keep a record array of a certain variable, but whenever I push that variable it doesn't push an instance of that variable, but the variable itself.

var array = [0,0,0];
var record = [];

record.push(array);
for(var i=0;i<array.length;i++) {
    array[i]++;
}
record.push(array);
console.log(record);
//I need record to be [[0,0,0],[1,1,1]], but it gives two copies of the latter

It's not pushing the variable. It's pushing the array that the variable references. You only created two arrays in your code and one of them is `record`. If you want each row of record to be another array you need to create an array for every row. You could do that by copying `array` (with `array.slice(0)` for example), or by using another array literal in your loop and then filling it with the correct values. — Paul, Oct 22 '18 at 01:32

Neii · Accepted Answer · 2018-10-22T06:03:48.770

1

You can use "slice" on the array (with optional "0" as first argument) to get a clone of it:

var array = [0,0,0];
var record = [];

record.push(array.slice(0));

for(var i=0;i<array.length;i++) {
    array[i]++;
}

record.push(array.slice(0));

console.log(record);
// record is: [[0, 0, 0], [1, 1, 1]]

edited Oct 22 '18 at 06:03

answered Oct 22 '18 at 01:32

Neii

598
2
7

1

Just `.slice()` works (and is the most common usage), no need for `.slice(0)`. – Pedro A Oct 22 '18 at 02:06

score 0 · Answer 2 · answered Oct 22 '18 at 01:33

0

You can use a built-in function to do that:

var record = array.slice(0);

ref: https://davidwalsh.name/javascript-clone-array

answered Oct 22 '18 at 01:33

Nikolay Vetrov

624
6
17

score 0 · Answer 3 · edited Jun 20 '20 at 09:12

Clone an Array

Array.slice is a more expensive operation (see this method comparison).
Instead, you can do something like so:

const array = [0,0,0];
const record = [Object.values(array)];

for(var i = 0; i < array.length; i++) {
    array[i]++;
}

record.push(array);

console.log(record);

UPDATE:
In order to make a copy of an array, there are many options that I can think of:

1) [...arr]
2) [].concat(arr);
3) arr.slice();
4) arr.map(i => i);
5) Array.of(...arr);
6) Object.values(arr);
7) Object.assign([], arr);
8) JSON.parse(JSON.stringify(arr));
etc...

snapshot of a variable javascript

3 Answers3

Clone an Array