Mutations, Rest Parameter, and Spread Operator

Mutations

• Reference data types like objects, arrays, and functions assigned to variables let and const are mutable.
  • But, primitive data types like boolean, number, string, etc... are not mutable (immutable)
• So, immutability means we want to preserve/keep our current state and mutating means the opposite.

const x = [1, 2, 3];
x[1] = 30;
console.log(x); //will display [1, 30, 3] cause we mutated index [1], changing 2 to 30.

• Note: Object.freeze(param) will prevent mutation.

Rest Parameter

• With ES6 came the rest parameter for functions.
• With this, we can now create more flexible functions that can take a variable amount of arguments, which are stored in an array.

const sum = (...args) => {
    return args.reduce((x, y) => x + y, 0); 
    // 0 is just the starter value
    // reduce creates one value from multiple args in this situation.
}
console.log(sum(5, 10, 20)); // 35

Spread Operator

• ES6 came out with the ... notation, and can be used with rest parameters and spread operators.
  • With rest parameters, the ... means it will collect "the rest" of the elements into an array (that's why it has to be the last arg).
  • With spread operators, the ... means that arrays, objects, and strings can be turned into single arguments/elements.

const food = ["Pizza", "Sushi", "Fruit"];
const moreFood = ["Wings", "Fish", ...food];
console.log(moreFood) = 
// Wings, Fish, Pizza, Sushi, Fruit