In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods or properties. There are 7 primitive data types: string, number, bigint, boolean, undefined, symbol, and null.

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.


Autoboxing: primitive wrapper objects in JavaScript

Due to a feature known as "autoboxing", JavaScript primitives appear to have methods and properties. For example, below it looks like toUpperCase() and length are methods and properties of the string primitive.

let mystring = "lower case string";
console.log(mystring.toUpperCase()); // 'LOWER CASE STRING'
console.log(mystring); // Still 'lower case string' as toUpperCase() didn't modify mystring.
// 17

What actually happens is that a wrapper object associated with the primitive is automatically accessed instead. 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.
  • BigInt for the bigint primitive.
  • Boolean for the boolean primitive.
  • Symbol for the symbol primitive.

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

Primitives are immutable

This example will help you understand that primitive values are immutable. A primitive can be replaced, but it can't be directly altered.

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

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

// By contrast, using an array method mutates the array
let foo = [];
console.log(foo);               // []
console.log(foo);               // ["plugh"]

See also