Splice also affecting initial array

Question

I want to split my array in half and save the results of that in another array, But without affecting the original.

So if I had [1,3,9,5] I would want to save it in a variable. Then I would create new array and copy the initial one in it. Then I would split that new array in half.

Meaning in the end I would have 2 array like this

initial [1,3,9,5]
halved [1,3]

The problem is that initial one is also splitted and I get 2 array with each one holding half the values.

var initial = [1,3,9,5];

var half = initial;

half = half.splice(0, Math.floor(half.length / 2));

console.log(initial);
console.log(half);

splice is destructive, slice is not. I learned it the hard way. — Sebastien Daniel
– Sebastien Daniel, Commented Mar 8, 2016 at 14:48
Omg. I guess a lot of answers here confused those two. I know I did. — Paran0a
– Paran0a, Commented Mar 8, 2016 at 14:52

Sebastian Simon · Accepted Answer · 2016-03-08 14:42:10Z

1

var half = initial;

copies the reference of initial to half. They’re the same array.

Either copy the values of the array with var half = initial.slice(); or get the half right away with

var initial = [1, 3, 9, 5];
var half = initial.slice(0, Math.floor(initial.length / 2));

answered Mar 8, 2016 at 14:42

Sebastian Simon

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Comments

The Process · Accepted Answer · 2016-03-08 14:50:49Z

1

You can do it like this

var initial = [1,3,9,5];

var half = initial.slice(0, Math.floor(initial.length / 2));

console.log(initial); 
console.log(half);

edited Mar 8, 2016 at 14:50

answered Mar 8, 2016 at 14:41

The Process

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Comments

Dmitri Pavlutin · Accepted Answer · 2016-03-08 14:51:12Z

1

Check the following:

Option 1 Creating a copy of the initial array

var initial = [1,3,9,5];

var half = [].concat(initial);

half = half.splice(0, Math.floor(half.length / 2));

console.log(initial);
console.log(half);

The [].concat(initial) allows to create a copy of the initial array.

Option 2 Do not modify the initial array, but use slice method

var initial = [1,3,9,5];

var half = initial.slice(0, Math.floor(half.length / 2));

console.log(initial);
console.log(half);

edited Mar 8, 2016 at 14:51

answered Mar 8, 2016 at 14:40

Dmitri Pavlutin

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2 Comments

Sebastien Daniel Over a year ago

While your option 1 works... it feels like using a sledge-hammer when a regular hammer (slice) is most appropriate. By using concat + spliceyou're essentially mimicking slice but in a less efficient way.

Dmitri Pavlutin Over a year ago

@SebastienDaniel Agreed. The 2 options are showed for a better understanding on the problem. Sure slice() is the correct approach.

Sebastien Daniel · Accepted Answer · 2016-03-08 14:56:24Z

1

When splitting an array there are two prototype methods:

splice: a destructive version which affects the original array.
slice: a pure version, which returns a new array without affecting the array on which it operates.

var half = initial.slice(0,Math.floor(half.length / 2));

Just replace your use of splice with slice and you're good to go.

For more information, I invite you to consult the excellent MDN document for Array.splice and MDN document for Array.slice

answered Mar 8, 2016 at 14:56

Sebastien Daniel

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Comments

jdabrowski · Accepted Answer · 2016-03-08 14:39:56Z

0

When you do var half = initial; you only pass a reference, not copy an array. You need to use other method to copy an array, for example:

var half = initial.slice();

answered Mar 8, 2016 at 14:39

jdabrowski

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