reduce() function

Also known as foldl, this method boils down a list of values into a single value.

Signature:

declare function reduce<T extends readonly [], Acc>(f: Arrow, seed: Acc, iterable: T): Acc;

Example

You can reduce values into homogeneous type.

const sum = (a:number, b:number) => a + b;

// with implicit seed with first element
reduce(sum, [1, 2, 3, 4]); // 10

// with explicit seed
reduce(sum, 0, [1, 2, 3, 4]); // 10

You can reduce values into heterogeneous type.

// reduce { id: number; score: number; } to number
reduce((acc, value) => acc + value.score, 0, [
 { id: 0, score: 1 },
 { id: 5, score: 2 },
 { id: 9, score: 3 },
 { id: 3, score: 4 }
])

Omitting iterable will returns function, useful when using with pipe.

pipe(
 [1, 2, 3, 4],
 map(a => a + 10),
 filter(a => a % 2 === 0),
 reduce(sum),
); // 26

For backward compatibility, reduce can support partial lazy form. You may want to use reduceLazy to use seed.

await pipe(
 Promise.resolve([1, 2, 3, 4]),
 map((a) => a + 10),
 filter(a => a % 2 === 0),
 reduce(sum),
); // 26

// if you want to use asynchronous callback
await pipe(
 Promise.resolve([1, 2, 3, 4]),
 toAsync,
 map(async (a) => a + 10),
 filter(a => a % 2 === 0),
 reduce(sum),
); // 26

// with toAsync
await pipe(
 [Promise.resolve(1), Promise.resolve(2), Promise.resolve(3), Promise.resolve(4)],
 toAsync,
 map(a => a + 10),
 filter(a => a % 2 === 0),
 reduce(sum),
); // 26

Try It

see pipe, toAsync, map, filter

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