Reference and value

December 16, 2020

In javascript, when you assign value to a variable, it has two different ways to achieve that: assign value or assign reference.

The main difference between the two is that passing by value happens when assigning primitives while passing by reference when assigning objects.

value

let a=10
let b=a
a=11
console.log(a,b) //11, 10

When assign primitives such as string, number, boolean or symbol to a variable. In memory it works like below:

a=10, it generate a key value pair and save them into stack

key	value
a	10

b=a

key	value
a	10
b	10

a=11

key	value
a	11
b	10

reference

let a = {age:10}
let b = a
b.age = 11
console.log(a.age) // 11

But if you assign object to a variable, in memory it works like below:

let a = {age:10}, it save the reference address of the object instead of the value of the object in the stack

key	address
a	address1

and in the heap it save the object value:

address	value
address1	{age:10}

Therefore, when assign b=a, they will share the same reference address in the stack

key	address
a	address1
b	address1

and this is why when you change the value of b, it will also effect a

b.age = 11

address	value
address1	{age:11}

Why make reference instead of value?

It is because normally object could be huge, and if we all save them as key-value pairs in the stack, it will lead to a heavy memory cost.