Practice: Immutable vs. Mutable in JavaScript

Example

This example will help you understand that primitive values are immutable. Start by opening a Developer Console in your browser. The sequence is ctrl+shift+J on Windows and cmd+option+J on the Mac. Then, you'll use the "console.log" method to write output to the developer console.

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.