itertools

• Fix missing top-level exports for izipLongest3 and intersperse

• Actually export the new itertool at the top level

• Add new dupes(iterable, keyFn?) function, which returns groups of all duplicate items in iterable.

• Add missing export for repeat

• Type output types of all itertools more precisely

Fixes a bug where some iterators would render an inputted generator unusable, causing it to no longer be consumable after the iterable returns.

Example:

function* gen() {
  yield 1;
  yield 2;
  yield 3;
  yield 4;
}

const lazy = gen();

// [1, 2]
Array.from(islice(lazy, 0, 2));

Array.from(lazy);
// ❌ Previously:    []
// ✅ Now correctly: [3, 4]

This bug only happened when the source was a generator. It did not happen on a normal iterable.

Similar bugs were present in:

• find()
• islice()
• takewhile()
• dropwhile()

No other iterables were affected by this bug. This is the same bug that was fixed in 2.2.2 for reduce(), so many thanks again for surfacing this edge case, @quangloc99! 🙏

• Fix reduce() bug where using it on a lazy iterable would produce the wrong result (thanks for finding, @quangloc99 🙏!)

• Fix islice() regression where it wasn't stopping on infinite iterables (thanks for finding, @Kareem-Medhat 🙏!)

• Move to ESM by default
• Drop support for node 12.x and 14.x
  (they probably still work, but they're EoL)

Improve tree-shakability when used in bundlers.

• Improve documentation
• Fix a minor edge case bug in reduce() (when first argument in iterable is undefined, and the predicate function was not explicitly set)

• The following functions retain richer type information about their arguments:

  • ifilter()
  • filter()
  • partition()

  For example, TypeScript will now know the following:

  const items = [3, 'hi', -7, 'foo', 13];
  
  function isNum(value: unknown): value is number {
      return typeof value === 'number';
  }
  
  const numbers: number[] = filter(items, isNum); // ✅
  
  const [numbers, strings] = partition(items, isNum); // ✅
  //     ^^^^^^^  ^^^^^^^ string[]
  //     number[]
  

• Add new find(iterable, pred) function, which is almost the same as first(iterable, pred) but behaves slightly more intuitive in the case where no predicate function is given.

• Fix bug in chunked() with size=1

Breaking changes:

• Rewritten source code in TypeScript (instead of Flow)
• Modern ESM and CJS dual exports (fully tree-shakable when using ESM)
• Massively reduced bundle size
• Targeted ES2015 (instead of ES5)
• Support only TypeScript versions >= 4.3
• Drop Flow support[^1]
• Drop Node 10.x support
• icompact, compact, and compactObject functions will now also remove null values, not only undefined

[^1]: I'm still open to bundling Flow types within this package, but only if that can be supported in a maintenance-free way, for example by using a script that will generate *.flow files from TypeScript source files. If someone can add support for that, I'm open to pull requests! 🙏

Breaking changes:

• icompact, compact, and compactObject functions will now also remove
  null values, not only undefined
• Support only Node >= 12.x
• Support only TypeScript versions >= 4.3
• Drop Flow support

Changes:

• Modern ESM and CJS dual exports
• Rewrite the source code in TypeScript

• Add missing re-export of islice at the top level

• TypeScript support!
• Declare official support for Node 16.x
• Drop support for Node 13.x (unstable release)

• Include an error code with every FlowFixMe suppression (Flow 0.132.x compatibility)

• New itertool: heads()

• Export roundrobin() at the top level

• Fix bug in chunked() when input is exactly dividable