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[2] + myarray[1] + myarray[0];
var result4 = myarray[3] + myarray[2] + myarray[1] + myarray[0];

so all that would output 5, 15, 18, 20

but instead of writing out all the vars like that, I want it to say something like:

var result = arrayitem + the sum of any previous items

Does that make sense? Is that possible? How do I do that?

Borewit · Accepted Answer · 2025-08-04 07:17:03Z

0

I was looking for how to do cumulative sums within groupBy in using @tidyjs/tidy. Here is what works for me:

// Here is how to do a cumSum using tidy.js starting with unsorted data
async function runAsync() {

    // Dynamic import ESM @tidyjs/tidy in this CommonJS env
    const tidy = await import('https://cdn.jsdelivr.net/npm/@tidyjs/[email protected]/+esm');
    const { tidy: tidyFn, groupBy, sort, mutateWithSummary, roll, n, sum } = tidy;

    const data = [
        {str: 'foo', value: 3},
        {str: 'bar', value: 3},
        {str: 'foo', value: 1},
        {str: 'bar', value: 1},
        {str: 'bar', value: 7},
    ];
    const test = tidyFn(data,
        groupBy(['str'], [
                mutateWithSummary({
                    cumsum: roll(n(), sum('value'))
                })
            ]
        ));
    console.log("test: " + JSON.stringify(test));
}

// Here is how to do a cumSum using tidy.js starting with unsorted data
async function runAsync() {

    // Dynamic import ESM @tidyjs/tidy in this CommonJS env
    const tidy = await import('https://cdn.jsdelivr.net/npm/@tidyjs/[email protected]/+esm');
    const { tidy: tidyFn, groupBy, sort, mutateWithSummary, roll, n, sum } = tidy;

    const data = [
        {str: 'foo', value: 3},
        {str: 'bar', value: 3},
        {str: 'foo', value: 1},
        {str: 'bar', value: 1},
        {str: 'bar', value: 7},
    ];
    const test = tidyFn(data,
        groupBy(['str'], [
                mutateWithSummary({
                    cumsum: roll(n(), sum('value'))
                })
            ]
        ));
    console.log("test: " + JSON.stringify(test));
}

runAsync().catch(err => console.error('oops', err));

edited Aug 4 at 7:17

Borewit

1,9452 silver badges22 bronze badges

answered Jul 17 at 15:18

Peter Jackson

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6 Comments

Peter Jackson Jul 19 at 12:59

Thanks, Borewit, but I get an error when I try to run it in Stack Snippets: { "message": "SyntaxError: import declarations may only appear at top level of a module", "filename": "stacksnippets.net/js", "lineno": 13, "colno": 1 }

[deleted]

Peter Jackson Jul 21 at 0:17

Sorry. I could not get the async import to work.

Borewit Jul 23 at 6:53

Hi Peter, I made your code is now runnable. I deleted my previous comments as they are no longer relevant.

Peter Jackson Jul 24 at 12:37

Thanks, Borewit. That is instructive.

Borewit Aug 1 at 8:16

Peter, you broke the code by removing the last line: runAsync().catch(err => console.error('oops', err))

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