Collecting

A collector takes an iterable and returns a value, so it ends a pipeline. Collectors that need configuration are called with it, like reduce(fn, 0). The others go in a pipeline as is, like toArray.

toArray, toSet, toMap, and toObject

Collect the values into an array, a Set, a Map, or an object. toMap and toObject take [key, value] pairs. toObject makes an object without a prototype, so a key like __proto__ is just an ordinary property.

TypeScript

import * as Pact from "@hyperjump/pact";

Pact.toArray(new Set([1, 2])); // [1, 2]
Pact.toSet([1, 1, 2]); // new Set([1, 2])

const pairs: [string, number][] = [["a", 1], ["b", 2]];
Pact.toMap(pairs); // new Map([["a", 1], ["b", 2]])
Pact.toObject(pairs); // { a: 1, b: 2 }

reduce

Calls a function with the accumulated value and each value in turn. It returns the final accumulated value. Without an initial value, the first value is used as the initial value. In that case an empty iterable throws a TypeError, just like Array.prototype.reduce.

TypeScript

import * as Pact from "@hyperjump/pact";

const sum = (total: number, n: number) => total + n;

Pact.reduce(sum, 10)([1, 2, 3]); // 16
Pact.reduce(sum)([1, 2, 3]); // 6

every, some, and find

• every checks whether a function returns true for every value.
• some checks whether it returns true for any value.
• find returns the first value it returns true for, or undefined.

Each one stops reading and closes the iterable as soon as it knows the answer.

TypeScript

import * as Pact from "@hyperjump/pact";

Pact.every((n: number) => n > 0)([1, 2, 3]); // true
Pact.some((n: number) => n > 2)([1, 2, 3]); // true
Pact.find((n: number) => n > 1)([1, 2, 3]); // 2

forEach

Calls a function with each value and returns undefined. Use it for side effects at the end of a pipeline. To look at values in the middle of a pipeline, use tap.

TypeScript

import * as Pact from "@hyperjump/pact";

Pact.pipe(
  ["a.json", "b.txt"],
  Pact.filter((file) => file.endsWith(".json")),
  Pact.forEach((file) => console.log(file))
);

Output

a.json

head and count

head returns the first value and then closes the iterator. If there are no values, it returns undefined. count returns how many values there are.

TypeScript

import * as Pact from "@hyperjump/pact";

Pact.head(Pact.range(5)); // 5
Pact.count(Pact.range(0, 5)); // 5

join

Joins the values into a string with a separator between them.

TypeScript

import * as Pact from "@hyperjump/pact";

Pact.pipe(
  ["a", "b"],
  Pact.map((s) => s.toUpperCase()),
  Pact.join(", ")
); // "A, B"