How do you copy by value a composite data type in JavaScript ?

What is a Data type?

In computer science and computer programming, a data type or simply type is an attribute of data which tells the compiler or interpreter how the programmer intends to use the data.

Three major Categories of Data types in JavaScript.

  1. Primitive :data types such as number: 2,1.5, Boolean: True/false, String: ‘Apple’.

Copy by value Vs Copy by Reference in JavaScript.

What is copy by value?

In copy by value, When the variable data is assigned to another variable, both the variables refer to different memory locations. Here deep copying is performed, i.e if the value is altered in one variable it doesn’t affect the value stored in another variable as both the data are stored in different memory locations. All primitive and special data types are copied by value. For example.

What is copy by reference?

In copy by reference, When the variable data is assigned to another variable, only a new address for the assigned variable is created, i.e If the data of one variable is altered it also changes the data in another variable, as the data from both the variable are stored in same memory location(i.e both the address of the variable point to same memory location).All composite data types are copied by reference. For example.

So now arises a question, Can you copy by value a composite data type in JavaScript ?

Yes you can, there are basically five ways to do it.

  1. Using spread operator(…).

1.Using Spread

Spread operator is used to spread elements in an array into their individual property. For instance if there is a function and addition is to be performed between two variables and if it’s values are in the form of an array, then by using spread operator, it spreads the elements of the array into their individual datatype(here i.e number)and performs the addition. As shown in the example below.

This ability of Spread operator to spreads the elements of the array into their individual datatype, helps us to clone the object and copy the elements by value. In the example below when copied variable value is changed but original variable value remain same.

2. Using Object.assign()

The Object.assign() method copies all elements own properties from one or more source objects to a target object. Note the empty [] as the first argument, this will ensure you don’t mutate the original object.

In the above example when the variable b is assigned to a, and the value of b is altered, the corresponding index value of a also altered. But when Object.assign() is used, a clone of b is made and values altered in this clone does not affect the values in a.

3. Using JSON.parse() and JSON.stringify()

The JSON object, available in all modern browsers, has two useful methods to deal with JSON-formatted content: parse and stringify.

JSON.stringify() takes a JavaScript object and transforms it into a JSON.

JSON.parse() takes a JSON string and transforms it into a JavaScript object.

so we can use it here, first convert the object to string using stringify() method and then convert it back to object using parse().

4.Using slice()

This method is normally used to return a subset of the elements, starting at a specific index and optionally ending at a specific index of the original array. When using array.slice()you will end up with a copy of the original array.


This methods will return a new array with all (or some) values of the original one. While doing that, you can also modify the values, which comes in very handy: