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.
For example, below it looks like
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. console.log(mystring.length); // 17
What actually happens is that a wrapper object associated with the primitive is automatically accessed instead.
undefined, all primitive values have object equivalents that wrap around the primitive values:
Stringfor the string primitive.
Numberfor the number primitive.
BigIntfor the bigint primitive.
Booleanfor the boolean primitive.
Symbolfor the symbol primitive.
valueOf() method returns the primitive value.
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 bar.toUpperCase(); 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); //  foo.push("plugh"); console.log(foo); // ["plugh"]