Creating an array of cumulative sum in javascript

Question

This is an example of what I need to do:

var myarray = [5, 10, 3, 2];

var result1 = myarray[0];
var result2 = myarray[1] + myarray[0];
var result3 = myarray         


        

Answer 1

use reduce to build the result directly and non-destructively.

a.reduce(function(r,c,i){ r.push((r[i-1] || 0) + c); return r }, [] );

Answer 2

Simple solution using for loop

var myarray = [5, 10, 3, 2];

var output = [];
var sum = 0;

for(var i in myarray){
  sum=sum+myarray[i];
  output.push(sum)
}
console.log(output)

https://jsfiddle.net/p31p877q/1/

Answer 3

Simple solution using ES6

let myarray = [5, 10, 3, 2];
    let new_array = [];  
    myarray.reduce( (prev, curr,i) =>  new_array[i] = prev + curr , 0 )
    console.log(new_array);

For more information Array.reduce()

Arrow function

Answer 4

I needed to keep the results and just add a running total property. I had a json object with a date and revenue and wanted to display a running total as well.

//i'm calculating a running total of revenue, here's some sample data
let a = [
  {"date":  "\/Date(1604552400000)\/","revenue":  100000 },
  {"date":  "\/Date(1604203200000)\/","revenue":  200000 },
  {"date":  "\/Date(1604466000000)\/","revenue":  125000 },
  {"date":  "\/Date(1604293200000)\/","revenue":  400000 },
  {"date":  "\/Date(1604379600000)\/","revenue":  150000 }
];

//outside accumulator to hold the running total
let c = 0;

//new obj to hold results with running total
let b = a
  .map( x => ({...x,"rtotal":c+=x.revenue}) )
  
//show results, use console.table if in a browser console
console.log(b)

Answer 5

A simple function using array-reduce.

const arr = [6, 3, -2, 4, -1, 0, -5];

const prefixSum = (arr) => {

  let result = [arr[0]]; // The first digit in the array don't change
  arr.reduce((accumulator, current) => {
       result.push(accumulator + current);

       return accumulator + current; // update accumulator
  });
  return result;
}

Answer 6

A more generic (and efficient) solution:

Array.prototype.accumulate = function(fn) {
    var r = [this[0]];
    for (var i = 1; i < this.length; i++)
        r.push(fn(r[i - 1], this[i]));
    return r;
}

or

Array.prototype.accumulate = function(fn) {
    var r = [this[0]];
    this.reduce(function(a, b) {
        return r[r.length] = fn(a, b);
    });
    return r;
}

and then

r = [5, 10, 3, 2].accumulate(function(x, y) { return x + y })