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Javascript by reference vs. by value [duplicate] 4 answers

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Primitive data types (number, string and boolean) do not change if you change a reference of it, whereas composite data types do change.

http://msdn.microsoft.com/en-us/library/ie/7wkd9z69(v=vs.94).aspx

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In javascript Arrays & Objects are passed by reference, so changing them one place affects others.

And primitives like number , string are passed by value, so changing them at one place doesn't affect on others.

Primitives

var a,b;
a=10;
b=a;

So b & a has something like bellow structure

a ---1000----> [10] a is pointing to some location(lets 1000) which have value 10
b ---1004----> [10] b is pointing to some location(lets 1004) which have value 10

Lets we increment a by 1 now the value will be changed at the place 1000 .

a ---1000----> [11]
b ---1004----> [10]

And in Arrays and Objects

obj1 = {}; // obj1 has a reference

obj1 has an structure like bellow

------->1000--------->[{}]
obj1 -------1004----->[1000] //this '1000' is a reference which has a `{}` at it's place 

This line

obj2 = obj1;

after this line obj2 & obj share same reference ------->1000--------->[{}] obj1 -------1004----->[1000] obj2 -------1008----->[1000]

obj1.ab = 5; 

this line is adding a field called ab to the reference of obj1

------->1000--------->[{ab:5}]
obj1 -------1004----->[1000]
obj2 -------1008----->[1000]

And because obj1 & obj2 have same reference, you are getting the field ab for both.

obj1      // Object {ab: 5}
obj2      // Object {ab: 5}

Note:- Any improvement in answer is appreciated.

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arr1 contain a reference for the empty list. Then arr3 receives a reference for the same list, and arr2 a reference for the same list. So when you make arr1.ab = 5, you added a item in that list named 'ab' and gave the value 5. But since arr2 and arr3 point to the same list, you will get the same value.

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