Iterator Helpers Polyfill

This is a dependenciless ES2018+ polyfill for Iterator Helpers Proposal.
The polyfill aims to be as specification-conformant as possible.
NOTE: By default this package is pure: to add the Iterator and AsyncIterator as globals please use the installIntoGlobal function to add them into globalThis.
NOTE By default Iterator.range and AsyncIterator.range are not inclusive in contrary to what was shown in the previous version of examples and this has been fixed and new inclusive parameter is introduced to control that.

Method Documentation

Polyfilled methods

These methods were polyfilled word-to-word as specified in the iterator helpers proposal.

.map(mapper);

Transforms each this iterator value by calling passed mapper function.
Example:

Iterator.range(1, 5, 1, true).map(x => x * 2).toArray(); // [2, 4, 6, 8, 10]

.filter(filterer);

Filters each value out from this iterator if called filterer function returns false for this value.
Example:

Iterator.range(1, 10, 1, true).filter(x => x % 2 === 1).toArray(); // [1, 3, 5, 7, 9]

.asIndexedPairs();

Aliases: .enumerate(); .entries();
Transforn each value of this iterator into array of value's position index in the iterator and value itself.
Example:

Iterator.range(1, 10, 1, true).map(x => Math.random() * 256 | x).asIndexedPairs().toArray() // [[0, 153], [1, 94], ...]

.drop(times: number);

Aliases: .skip();
Drops values from this iterator passed number of times (throws error if negative).
Example:

Iterator.range(1, 10, 1, true).drop(5).toArray(); // [6, 7, 8, 9, 10]

.take(times: number);

Takes values from this iterator passed number of times (throws error if negative).
Example:

Iterator.range(1, 10, 1, true).take(5).toArray(); // [1, 2, 3, 4, 5]

.every(fn);

Returns true if this iterator is empty or every value of it satisfies function fn.
Examples:

Iterator.range(2, 10, 1, true).every(x => x % 2 === 0); // false
Iterator.range(1, 10, 1, true).every(x => x > 0); // true
Iterator.from().every(() => false); // true

.find(finder);

Returns first value of this iterator that satisfies function finder.
Example:

Iterator.range(1, 10, 1, true).find(x => x % 2 === 0) // 2

.flatMap(mapper);

Yields values of each transformed values of this iterator by calling mapper (throws when mapper function throws non-iterable/iterator value).
Examples:

Iterator.range(1, 4, 1, true).flatMap(x => `${ x }`.repeat(x)).toArray(); // ["1", "2", "2", "3", "3", "3", "4", "4", "4", "4"]
Iterator.range(1, 5, 1, true).flatMap(function* (x) {
    yield x ** 2;
    yield x * 10;
}).toArray(); // [1, 10, 4, 20, 9, 30, 16, 40, 25, 50]

.forEach(mapper);

Calles mapper function with each value of this iterator and returns undefined.
Example:

Iterator.range(1, 5, 1, true).forEach(console.log);
// Console output:
// 1
// 2
// 3
// 4
// 5

.reduce(reducer, initialValue?);

TLDR: basically same as Array.prototype.reduce().
Firstly, this method creates an accumulator variable for later use from initialValue parameter if it's specified or gets first value of this iterator or throws if neither of these were the case. Then it takes following values and calls reducer function with them and accumulator and then assigns result of the call to the accumulator variable.
Examples:

Iterator.range(1, 10, 1, true).reduce((accumulator, x) => accumulator + x); // 55
["This", "is", "interesting!"].values().reduce((accumulator, x) => `${ accumulator } ${ x }`); // "This is interesting!"
["this", "is", "also", "interesting!"].values().reduce((accumulator, x) => `${ accumulator } ${ x }`, "And also i know that"); // "And i also know that this is interesting!"
Iterator.from().reduce(() => { throw "Never happens!" }, 1); // 1
Iterator.from().reduce(() => {}); // This throws an error since iterator is empty and no `initialValue` parameter is specified.
Iterator.from().reduce(() => {}, undefined); // But this works since `initialValue` parameter is set to the undefined value; undefined

.some(filterer);

Returns true if this iterator is empty or also returns true if some values of this satisfy condition function filterer or returns false otherwise.
Examples:

Iterator.range(1, 10, 1, true).some(x => x === 5); // true
Iterator.from().some(() => {}); // true

.toArray();

Returns array containing all values of this iterator.
Example:

Iterator.range(1, 5, 1, true).toArray(); // [1, 2, 3, 4, 5]

Iterator.from(value);

Returns a iterator if value is a valid iterable (if iterator is with %Iterator.prototype% in the prototype chain) or wraps invalid iterator into %WrapForValidIteratorPrototype%.
Examples:

Iterator.from([0, 1, 2, 3, 4]); // same as [0, 1, 2, 3, 4][Symbol.iterator]();
Iterator.from({ next() { return { done: true, value: undefined } } }); // wrapped into %WrapForValidIteratorPrototype%.

Additional methods

.max();

Returns maximum number found in this iterator.
Example:

Iterator.from([0, -9, 23, 96, 10, 92]).max(); // 96

.max();

Returns minimal number found in this iterator.
Example:

Iterator.from([0, -9, 23, 96, 10, 92]).min(); // -9

.average();

Returns average number from all the numbers from this iterator.
Example:

Iterator.from([10, 50, 40, 20]).average(); // 30

.chain(...iterators);

Returns an iterator which yields values sequencially of this iterator first and then all the iterators one by one left to right.
Example:

Iterator.range(1, 3, 1, true).chain(Iterator.range(4, 6, 1, true), Iteratior.range(7, 10, 1, true)).toArray; // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

.join(separator: string = ", ");

TLDR: Same as Array.prototype.join() but default separator is ", ", not ",".
Returns a string of all values of this iterator stringified and joined by seperator.
Example:

Iterator.range(1, 5, 1, true).join(); // "1, 2, 3, 4, 5"

.cycle(times: number = Infinity);

Aliases: .repeat() Repeats values of this iterator passed number of times or infinite amount of times if nothing is passed as times argument.
Example:

Iterator.range(1, 3, 1, true).cycle(2).toArray(); // [1, 2, 3, 1, 2, 3]

(Async)Iterator.range(start: number, end: number = (_ => (start = 0, _))(start), step: number = 1, inclusive: boolean = false)

Produces range of number values from start to end incremented by step until start is greater than (inclusive == false, default) / greater than or equal to (inclusive == true) end if step is positive and not zero, othewrwise until start is less than (inclusive == false) or less than or equal to (inclusive == true) end Example:

// inclusive
Iterator.range(1, 3, 1, true).toArray(); // [1, 2, 3]
// exclusive
Iterator.range(1, 3, 1).toArray(); // [1, 2]
// inclusive reverse
Iterator.range(-1, -3, -1, true).toArray(); // [-1, -2, -3]
// exclusive
Iterator.range(-1, -3, -1).toArray(); // [-1, -2]
// edge cases
Iterator.range(1, 1, 0, false).toArray(); // []
Iterator.range(0, 1, 0, true).toArray(); // []
Iterator.range(1, 0, 0, false).take(2).toArray(); // [1, 1], .range() itetates infinitely if not in bounds by .take(2), never finishing
Iterator.range(0, 0, 0, true).take(2).toArray(); // [0, 0], .range() itetates infinitely if not in bounds by .take(2), never finisheing

Notes:

• It also implements (Async)Iterator.prototype.flatMap() behavior from issue #114 - flatMap should act like it does a yield * on each iterable.
• Pre-checks are done BEFORE all functions start doing their stuff, INCLUDING async ones. (Yet i have unsettled thinking about async function pre-checks, maybe variations of pre-check will come out behind a flag? (in the config variable)).
• It takes globalThis by a polyfill written by Mathias Bynens from his awensome article (Lingers on fact that __0x_6642_5d0e_72c2_4e09 preperty is writable and extensible in Object.prototype and Object.defineProperty isn't changed before the script is run).
• UMD bundle is exposed into global as __IteratorHelpersPolyfill.
• All bundles are minified.
• Additional helpers can be removed by the config variable from exports.
• This polyfill's size is less than 5 kb (4.62 kb atm) when compressed by Brotli compression algorithm.
• Typings are made to accurately reflect behaviour of methods as much as possible.

Development:

Please propose feature by opening an issue before starting working on pull request, because i can reject your proposal and your work won't be needed at all

1. Fork & Clone forked repository to your local machine.
2. Open cloned directory in your favorite editor.
3. Do some commits.
4. Open pull request!

Build:

1. Install all modules by npm i in the command prompt.
2. Build by npm run build if you just need the .mjs bundle or npm run build-full if you also need commonjs and umd bundle.
   NOTE: Builds include source maps.