mutable-vs-immutable.md

#Mutable vs immutable types

In javascript we have primitive types and reference types, which correspond to immutable and mutable types.

##Primitive Types

• Number: 14, 4.12
• Boolean: _true, false
• String: "Hello world"
• null
• undefined

##Reference Types

• Object:

{
  name: "Ben",
  age: 20
}

• Array: [23, 4, 55, 9]
• Function: myFunction()

##Mutability

Reference types in Javascript are mutable. Consider this example:

var ben = {
  age: 20
};

var another = ben; //ben is assigned to a new variable called another

console.log(ben.age); //prints 20
console.log(another.age); //prints 20

another.age = 35; //another's age is set to 35

console.log(ben.age); //prints 35
console.log(another.age); //prints 35

Notice that when we modify another, ben is also modified (or mutated).

Both variables are acting on the same object, so changing one will affect the other.

When this code runs:

var another = ben;

... the variable another is NOT assigned a copy of ben. It points to the same object as ben.

An example with a function call:

function modifyAge(obj) {
	obj.age = 10;
}

var sally = {
  age: 23
};

modifyAge(sally);

console.log(sally.age); //prints 10

When we pass sally to modifyAge(), we are not passing a copy of sally which is why when we print sally after calling modifyAge(), we can see how the original object has been updated.

##Immutability

The case is different for primitive types. Primitive types are immutable, they CANNOT be modified.

var a = 5;

var b = a;

console.log(a); //prints 5
console.log(b); //prints 5

a++;

console.log(a); //prints 6
console.log(b); //prints 5

As opposed to what happens with reference types, when we change the value of a it has no effect on the variable b.

There is no change that can be applied to a that will affect b and vice versa. There is no connection between the two.

An example with a function call:

function modifyValue(n) {
	n++;
}

var age = 53;

modifyValue(age);

console.log(age); //prints 53

In this example, an immutable type (a Number) is passed to the function modifyValue(). There, it is called n. But when n is modified, it has no effect on the original age variable.