Primitive vs complex data structures

data.js

// string is a primitive data type
const name = 'Michael';
name.toUpperCase();
// toUpperCase is not a method; it does not mutate
// the original variable
console.log(name); // Michael

// instead, it returns a new one
const NAME = name.toUpperCase();
console.log(NAME); // MICHAEL

// list is a complex data type
const names = ['Alice'];
// push mutates the original array
names.push('Bob');
console.log(names); // ['Alice', 'Bob']