CODEWITHSUNDEEP
javascript
Richard
Richard

Reputation: 65510

Copying array variables in JavaScript?

Please could someone tell me why this operation changes the value of the original array?

var today = new Date(); 
var max_x_domain = [today];

var one_year_after_end = max_x_domain.slice(0)[0];
one_year_after_end.setYear(one_year_after_end.getFullYear() - 1);

console.log('array after operation', max_x_domain);

And how can I make a copy of max_x_domain[1]... do I need to use a clone method as described here?

This is... a surprising feature of JavaScript.

Upvotes: 1

Views: 103

Answers (3)

japrescott
japrescott

Reputation: 5015

when using sliceyou create a copy and therefor a new array, without modifying the original. But the Elements inside are not copied/cloned, unless they are of a primitive type (bool, string, number). Since you have a Date Object in there, there will be a new array created but the same reference to your date object will be 'copied'. Thus, if you want to have an fresh Date Object, do new Date(oldDate) / var one_year_after_end=new Date(max_x_domain.slice(0)[0]) and you can modify it from there on without modifying the original Date Object.

Upvotes: 1

Anthony Grist
Anthony Grist

Reputation: 38345

It's not at all surprising if you consider that your array contains object references. As this section of the MDN docs states:

For object references (and not the actual object), slice copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.

Upvotes: 3

Alcides Queiroz
Alcides Queiroz

Reputation: 9576

var dateToCopy = new Date();

var newDate = new Date(dateToCopy);

Refactored code

var today = new Date(); 
var max_x_domain = [today];

var one_year_after_end = new Date(max_x_domain.slice(0)[0]);
one_year_after_end.setYear(one_year_after_end.getFullYear() - 1);

console.log('array after operation', max_x_domain);

Upvotes: 1

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