scan doesn't accept undefined as seed value #2047

derekrjones · 2016-10-17T20:32:22Z

problem: scan(reduceFn, undefined) behaves identically to scan(reduceFn)
proposal: undefined is a valid seed value

(1) it is not consistent with Array.prototype.reduce

['a', 'b', 'c'].reduce(function(acc, s){ return acc + ' ' + s; })
"a b c"
['a', 'b', 'c'].reduce(function(acc, s){ return acc + ' ' + s; }, undefined)
"undefined a b c"

(2) with certain kinds of reducing functions, this behavior can lead to unexpected behavior

var reducer = new Rx.Subject();
reducer.scan(function(acc, fn) { return fn(acc); }, initialState);
reducer.next(function nextState(state){ /* return new state */ })

if initialState is undefined, then subject will emit nextState function instead of nextState(undefined) leading to undesired behavior. Using a different value for initialState could also result in undesired behavior for example if combined with distinctUntilChanged

the scan api doc has no mention that undefined is not a valid seed value

Returns an Observable that applies a specified accumulator function to each item emitted by the source Observable. If a seed value is specified, then that value will be used as the initial value for the accumulator. If no seed value is specified, the first item of the source is used as the seed.

The text was updated successfully, but these errors were encountered:

…ed value Array#scan supports `undefined` as a valid seed value, so we should too. ```js of(1, 2, 3).scan((acc, x) => acc + ' ' + x); // "undefined 1" // "undefined 1 2" // "undefined 1 2 3" ``` fixes ReactiveX#2047

…ed value Array#scan supports `undefined` as a valid seed value, so we should too. ```js of(1, 2, 3).scan((acc, x) => acc + ' ' + x, undefined); // "undefined 1" // "undefined 1 2" // "undefined 1 2 3" ``` fixes ReactiveX#2047

…ed value Array#reduce supports `undefined` as a valid seed value, so it seems natural that we would too for scan ```js of(1, 2, 3).scan((acc, x) => acc + ' ' + x, undefined); // "undefined 1" // "undefined 1 2" // "undefined 1 2 3" ``` fixes ReactiveX#2047

…itself as a valid seed (#2050) * fix(scan): scan operator now accepts `undefined` itself as a valid seed value Array#reduce supports `undefined` as a valid seed value, so it seems natural that we would too for scan ```js of(1, 2, 3).scan((acc, x) => acc + ' ' + x, undefined); // "undefined 1" // "undefined 1 2" // "undefined 1 2 3" ``` fixes #2047 * fix(reduce): reduce operator now accepts `undefined` itself as a valid seed value Array#reduce supports `undefined` as a valid seed value, so it seems natural that we would too. ```js of(1, 2, 3).reduce((acc, x) => acc + ' ' + x, undefined); // "undefined 1 2 3" ```

lock · 2018-06-06T23:40:34Z

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.

benlesh added the bug Confirmed bug label Oct 18, 2016

benlesh self-assigned this Oct 18, 2016

jayphelps mentioned this issue Oct 18, 2016

fix(reduce/scan): both scan/reduce operators now accepts undefined itself as a valid seed #2050

Merged

jayphelps closed this as completed in #2050 Oct 24, 2016

lock bot locked as resolved and limited conversation to collaborators Jun 6, 2018

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scan doesn't accept undefined as seed value #2047

scan doesn't accept undefined as seed value #2047

derekrjones commented Oct 17, 2016

lock bot commented Jun 6, 2018

scan doesn't accept undefined as seed value #2047

scan doesn't accept undefined as seed value #2047

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derekrjones commented Oct 17, 2016

lock bot commented Jun 6, 2018