Creating an array of cumulative sum in javascript

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An elegant solution copied from Nina Scholz, using currying to access the previous value. 

const cumulativeSum = (sum => value => sum += value)(0);

console.log([5, 10, 3, 2].map(cumulativeSum));

cumulativeSum is the function value => sum += value, with sum initialized to zero. Every time it’s called, sum is updated and will equal the previous value (output[n-1]) when called the next time (with input[n]).

Note that sum will need to be set to zero explicitly when you want to reuse the summation. The most convenient way to handle this may be to just inline the code instead of using a named function.

console.log([5, 10, 3, 2].map((sum => value => sum += value)(0)));
console.log([6, 10, 3, 2].map((sum => value => sum += value)(0)));

vs the unwanted behavior 

const cumulativeSum = (sum => value => sum += value)(0);

console.log([5, 10, 3, 2].map(cumulativeSum));
console.log([6, 10, 3, 2].map(cumulativeSum));

Update 7.6.2022:
You can use the comma operator instead of currying to make an easier-to-read version with explicit sum = 0. 

console.log([5, 10, 3, 2].map((sum = 0, n => sum += n)));

Three years and no one thought to suggest this? 🙂

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