up vote 0 down vote favorite

Just look at the code and you'll understand what I mean:

​var aBackup = [3, 4]; // backup array
​​var a = aBackup; // array to work with is set to backup array
a[0]--; // working with array..
a = aBackup; // array o work with will be rested
console.log(a);  // returns [2, 4] but should return [3, 4]
console.log(aBackup);​ // returns [2, 4] too​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​  but should return [3, 4] too
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possible duplicate of Is there a method to clone an array in jQuery? -- despite the term jQuery in the title/question, the solutions are not jQuery related. – Felix Kling Mar 20 at 10:46
@FelixKling sorry I'vent seen this duplicate. Everyone should vote to close. – micha Mar 20 at 10:56
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4 Answers

up vote 3 down vote accepted

You need to make real copies of your Arrays instead of just using a reference:

var aBackup = [3, 4]; // backup array
var a = aBackup.slice(0); // "clones" the current state of aBackup into a
a[0]--; // working with array..
a = aBackup.slice(0); // "clones" the current state of aBackup into a 
console.log(a);  // returns [3, 4] 
console.log(aBackup); // returns [3, 4]

See MDN for documentation on the slice-method

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nice, beat me to it! :) – ChrisR Mar 20 at 10:48
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up vote 1 down vote

Doesn't javascript uses pointer for arrays ? Should ​​var a = aBackup; do a copy ? otherwise the results seems normal to me...

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up vote 1 down vote

An array is a reference type of object, so changes made to it will change the underlying value it points to, a and aBackup will point to the same value, and a change made to a will change aBackup aswell.

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up vote 1 down vote

It is because when you do this, you are not making a copy of the array, but infact a reference to the original array.

var a IS aBackup; // if you will

When you need to do is clone the backup array.

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