CODEWITHSUNDEEP
javascript
Sruthi k
Sruthi k

Reputation: 1

javascript array object assigned to normal

I tried the following code

var foo = [1,2];
var bar = foo;
bar[0] = 9;
console.log(foo[0],bar[0]);    //the result is 9,9

How foo[0] can be 9?Please let me know this am stucked.Thanks in advance.

Upvotes: -1

Views: 58

Answers (3)

slebetman
slebetman

Reputation: 114014

Arrays, objects and functions are references in javascript while numbers and strings are values. Therefore when you do:

var foo = [];
var bar = foo;

The variable bar now points to foo and both foo and bar refer to the same array.

If you want to copy an array, use Array.prototype.slice():

var bar = foo.slice();

Technically, the .slice() method is meant to be used to return a subarray of an array but if called without any arguments it returns the whole array. The important thing to realize is that .slice() returns a new array, not the original array. Which is why we use it to do a shallow copy of an array.

For objects you can use the Objcet.create() function:

var foo = {};
var bar = Object.create(foo);

Additional notes:

It should be noted that both .slice() and Object.create() creates shallow copies of the original. That is, if the array or object contains other arrays or objects then the contents are still references:

var foo = [[1,2],3];
var bar = foo.slice();

bar[1] = 100;
console.log(bar[1]); // 100
console.log(foo[1]); // 3

bar[0][0] = 200;
console.log(bar[0][0]); // 200
console.log(foo[0][0]); // also 200

If you want to do a deep copy you need to write a recursive function to build another array/object based on the original. Or if you don't mind a bit of temporarily wasted RAM you can convert them to JSON and back again:

var bar = JSON.parse(JSON.stringify(foo));

Or use a library that provides a deep-copy function like jQuery.

Upvotes: 1

Ole Sauffaus
Ole Sauffaus

Reputation: 534

  1. makes foo an array.
  2. makes bar an reference to foo
  3. changing bar will really change foo

Now foo IS bar ... and vice versa

Upvotes: 1

chaintng
chaintng

Reputation: 1343

In javascript, array are pass by reference. That means bar is a pointer to the same object that foo point to.

You need to add clone method when you want to assign the variable by passing its value like following code

var foo = [1,2];
var bar = foo.clone();
bar[0] = 9;
console.log(foo[0],bar[0]);    //the result is 1,9

Upvotes: 2

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