spread operator vs array.concat()

Question

What is the difference between spread operator and array.concat()

Answer 1

There is one very important difference between concat and push in that the former does not mutate the underlying array, requiring you to assign the result to the same or different array:

let things = ['a', 'b', 'c'];
let moreThings = ['d', 'e'];
things.concat(moreThings);
console.log(things); // [ 'a', 'b', 'c' ]
things.push(...moreThings);
console.log(things); // [ 'a', 'b', 'c', 'd', 'e' ]

I've seen bugs caused by the assumption that concat changes the array (talking for a friend ;).