CODEWITHSUNDEEP
javascriptarrays
TSR
TSR

Reputation: 20406

Array copy using slice is not working

I would expect that after using distances.slice(), I would have a copy of the array. But when I change an element of the new copy, it still changes the original array. Why?

var distances = [
  ['-1', '10', '-1', '31'],
  ['10', '-1', '10', '-1'],
  ['-1', '-1', '-1', '10'],
  ['15', '6', '-1', '-1']
];
for (var i = 0; i < 4; i++) {
  console.log(distances, 'distances pairstop');
  var sampleDistance = distances.slice()
  sampleDistance[0][2] = ['fooo']
}

Upvotes: 2

Views: 1170

Answers (2)

Redu
Redu

Reputation: 26161

To clone an array with sub array items with indefinite depth you need a general array cloning tool like;

Array.prototype.clone = function(){
  return this.map(e => Array.isArray(e) ? e.clone() : e);
};

So distances.clone() should just do the job fine.

Upvotes: 1

Yossi Vainshtein
Yossi Vainshtein

Reputation: 3985

Slice produces a shallow copy of the array. It says so in the docs:

The slice() method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included). The original array will not be modified.

Since distances is an array of arryas, sampleDistance will be a shallow copy, meaning it will hold refernces to the same 3 (one-dimentional) arrays. sampleDistance[0][2] = ['fooo'] changes the first one-dimentional array, which is referenced both by sampleDistance and distances.

hence your bug.

Upvotes: 5

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