I have this array of numbers in JS code:

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];

I need to sum the each number with the numbers before.

var cumulative12ArrOfNumbers = [10, 30, 60, 100, 150, 210, etc to 11 index... ];

What is the best way to do that in JS?

UPD : I'm newbie in JS

I have this array of numbers in JS code:

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];

I need to sum the each number with the numbers before.

var cumulative12ArrOfNumbers = [10, 30, 60, 100, 150, 210, etc to 11 index... ];

What is the best way to do that in JS?

UPD : I'm newbie in JS

• javascript
• arrays

• What the code do you want? I want the solution from you. I've got just these arrays – Max Gabderakhmanov Commented Jan 6, 2017 at 9:19
• Nope that's not how SO works. You need to e up with a specific problem, and an attempt at a solution. Then we troubleshoot your code. You just want us to write code for you. That won't fly. – dda Commented Jan 6, 2017 at 9:36
• 1 In case you wonder, the downvotes are because you don't show research efforts – Randy Commented Jan 6, 2017 at 9:40
• @dda thanks for the explanation – Max Gabderakhmanov Commented Jan 6, 2017 at 9:44

6 Answers 6

You could use Array#map and return the sum of the item and the last sum.

This proposal does not alter the original array.

var array = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120],
    result = array.map(function (a) {
        return this.last += a;
    }, { last: 0 });

console.log(result);

A version without thisArg and a closure

var array = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120],
    result = array.map(function (last) {
        return function (a) {
            return last += a;
        };
    }(0));

console.log(result);

ES6

var array = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120],
    result = array.map((last => a => last += a)(0));

console.log(result);

Simple and fast solution using a regular for loop(without any functions):

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];
for (var i = 0, len = arrOf12Numbers.length; i < len; i++) {
    if (i > 0) arrOf12Numbers[i] = arrOf12Numbers[i] + arrOf12Numbers[i - 1];
}
console.log(arrOf12Numbers);

And it's more than 2 times faster than approach with map() function(if you care about performance)

To get result as a new array use the following approach:

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120],
    result = arrOf12Numbers.slice();

for (var i = 0, len = result.length; i < len; i++) {
    if (i > 0) result[i] = result[i] + result[i - 1];
}
console.log(result);

The second apprroach is still about 2 times faster than map() function approach

This will do the work without altering the original array. You just have to pass your original array to the function.

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];
var myFunc = function(arrOf12Numbers) {
  var cumulative12ArrOfNumbers = [];
  cumulative12ArrOfNumbers[0] = arrOf12Numbers[0];
  for(i=1; i<arrOf12Numbers.length; i++) {
    cumulative12ArrOfNumbers[i] = arrOf12Numbers[i-1] + arrOf12Numbers[i];
  }
  console.log(cumulative12ArrOfNumbers);
};
myFunc(arrOf12Numbers);

This might help you.

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];

var result = [];

var sum = 0;

arrOf12Numbers.forEach(function (val, key) {
  sum += val;
  result.push(sum);
});

console.log(result);

var arr = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];
for (var i = 1; i < arr.length; i++) {
    sum = arr[i] + arr[i-1];
    arr[i] = sum;
}

It's not really so plicated, just a loop;

var arrOf12Numbers = [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120];
var output = [], sum;

for (var i = 0; i < arrOf12Numbers.length;i++)
{       
  output.push(i === 0 ? arrOf12Numbers[i] : output[i-1] + arrOf12Numbers[i])
}

console.log(output);