Mutable vs Immutable

• Mutable refers to objects that can be changed after they are created
• Immutable refers to objects that cannot be altered once they are created

let x = [...]

// fun() can alter values in `x`
// can return value
x.fun()  //Mutable

// something() can do something😎 without changing values in `x`
// can return value
x.something() //Immutable