Primitive

A primitive (primitive value, primitive data type) is data that is not an object and has no methods. In JavaScript, there are 6 primitive data types: string, number, boolean, null, undefined, symbol (new in ECMAScript 2015).

Most of the time, a primitive value is represented directly at the lowest level of the language implementation.

All primitives are immutable, i.e., they cannot be altered. It is important not to confuse a primitive itself with a variable assigned a primitive value. The variable may be reassigned a new value, but the existing value can not be changed in the ways that objects, arrays, and functions can be altered.

Example

This example would help you understand the fact that primitive values are immutable.

JavaScript

// Using a string method doesn't mutate the string
var bar = "baz";
console.log(bar);               // baz
bar.toUpperCase();
console.log(bar);               // baz

// Using an array method mutates the array
var foo = [];
console.log(foo);               // []
foo.push("plugh");
console.log(foo);               // ["plugh"]

// Assignment gives the primitive a new (not a mutated) value
bar = bar.toUpperCase();       // BAZ

A primitive can be replaced, but it can't be directly altered.

Another Example [ Step-by-step ]

The following example will help you going through how JavaScript deal with Primitives.

JavaScript

// The Primitive 
let foo = 5;

// Defining a function that should change the Primitive value
function addTwo(num) {
   num += 2;
}
// Another function trying to do the same thing
function addTwo_v2(foo) {
   foo += 2;
}

// Calling our first function while passing our Primitive as an argument
addTwo(foo);
// Getting the current Primitive value
console.log(foo);   // 5

// Trying again with our second function...
addTwo_v2(foo);
console.log(foo);   // 5

Did you expect it to be 7 instead of 5? If so, read how this code runs:

• For both the addTwo and addTwo_v2 functions calls, JavaScript looks up the value for the identifier foo. It correctly finds our variable instanciated with our first statement
• After finding it, JavaScript passes that argument to the functions as a parameter
• Before executing the statements inside the functions' bodies, JavaScript takes a copy of the original passed argument (which is a primitive) and creates a local copy. These copies, existing only inside the functions' scopes, are accessible via the identifiers we specified in the functions' definitions (num for addTwo, foo for addTwo_v2)
• Then, the functions' statements are exectued:
  • In the first function, a local num argument had been created. We are increasing its value by 2, not the original foo's value!
  • In the second function, a local foo argument had been created. We are increasing its value by 2, not the original (external) foo's value! Also, in this situation, the external foo variable cannot be accessed in any way. This is because of JavaScript's lexical scoping and the resulting variable shadowing. The local foo hides the external foo. For more information, see Closures.
• In conclusion, any changes inside our functions won't affect the ORIGINAL foo at all, as we are working on our copies of it

That's why Primitives are Immutable. Because, we aren't working on them directly, we took our copy and keep working with it, without touching the original one.

Primitive wrapper objects in JavaScript

Except for null and undefined, all primitive values have object equivalents that wrap around the primitive values:

• String for the string primitive.
• Number for the number primitive.
• Boolean for the Boolean primitive.
• Symbol for the Symbol primitive.

The wrapper's valueOf() method returns the primitive value.

Learn more

General knowledge

• Introduction to JavaScript data types
• Primitive data type on Wikipedia