Primitive

In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods. 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.

This example will help you understand that primitive values are immutable.

// 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.

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.

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