Iterable.reduce takes null as firstReduction if explicitly given.

[lukasbuenger]

What happened

I wrote a super simple higher order function for List.reduce in order for reduce to be composable in a Ramda context:

const reduce = reducer => initialReduction => list => list.reduce(reducer, initialReduction); 

With this style, if I wanted to concat a list of strings, according to the docs I'd simply have to explicitly give the null as firstReduction:

const concatToString = reduce((acc, val) => acc.concat(val))(null);

From the docs:

If initialReduction is not provided, or is null, the first item in the Iterable will be used.

Turns out, that null (or undefined for what it's worth) is still passed as initialReduction which leads to unwanted behavior or even to errors.

Cause of error

https://github.com/facebook/immutable-js/blob/master/src/IterableImpl.js#L206

The check on L206 only takes the number of passed arguments into account, otherwise blindly assigns reduction = firstReduction (L209).

Possible solutions

a. Fix the docs
b. Fix the check

How to reproduce

Premise

const reduce = reducer => initialReduction => list => list.reduce(reducer, initialReduction); 
const addValues = reduce((a, b) => a + b)(null);
const concatValues = reduce((a, b) => a.concat(b))(null);

const listOfNumbers = Immutable.List.of(1, 2, 3)
const listOfStrings = Immutable.List.of('Hunter', 'S.', 'Thompson');
const listOfLists = Immutable.List.of(
  Immutable.List.of(1, 2),
  Immutable.List.of(3, 4),
  Immutable.List.of(5, 6)
);

Cases

// Expected 6
// This works as expected as the numeric value of `null` is 0.
addValues(listOfNumbers);
// 6

// Expected 'HunterS.Thompon' 
addValues(listOfStrings);
// 'nullHunterS.Thompon'

// Expected 'HunterS.Thompon' 
concatValues(listOfStrings);
// Uncaught TypeError: Cannot read property 'concat' of null

// Expected [1, 2, 3, 4, 5, 6] 
concatValues(listOfLists).toJS();
// Uncaught TypeError: Cannot read property 'concat' of null

[leebyron]

Thanks for the report. The docs are out of date with the implementation which was updated to match the behavior of Array#reduce