The javascript

const arr = [10, 20, 3, 4, 5];
const newArr = [0, 1, -2];
const newArray = newArr.concat(arr); // or  Spread Operator
console.log(newArray);  // [ 0, 1, -2, 10, 20, 3, 4, 5 ]

const arr = [1, 2, 3];
arr[-1] = 0;

console.log(arr.length)

    let arr = [];
    if (arr) {
        alert('true');
    }