changeOfPosition.js

var data = [1, 3, 3, 4, 5, 3, 1];

function changeOfPosition(data) {
    var d = Array(data.length - 1);
    for (var i = 0; i < d.length; i++) {
	d[i] = data[i+1] - data[i];
    }
    return d;
}

console.log(data);
console.log(changeOfPosition(data));