JSFiddle - React, Tailwind, and code Playground

JavaScript

var A = [1, 2, 3, 3, 0];
function diff(ary) {
    var newA = [];
    for (var i = 1; i < ary.length; i++)  newA.push(A[i] - ary[i - 1])
    return newA;
}
alert(diff(A)) // [1, 1, 0, -3]