Array.prototype.reduce() on arrays of one element

Question

In following reduction + map operations, no. 3 is puzzling me. Can anyone please explain why

// 1
[1,2,3,4,5].filter(x => x==3).reduce((x, y) => y) // -> 3, all good

// 2
[1,2,3,4,5].filter(x => x<=3).reduce((x, y) => 0) // -> 0, still good

// 3
[1,2,3,4,5].filter(x => x==3).reduce((x, y) => 0) // -> 3, hello?

In other words: how come the reduction on the array of one element ignores the map to 0 operation? This would ultimately be used on an array of objects, as in .reduce((x,y) => y.attr) which also returns y instead of y.attr for single element arrays.

part of the spec: "If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callback." developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Daniel A. White
– Daniel A. White, Commented May 10, 2016 at 16:11
This obviously had to lead to some RTFM answer of sorts. Many thanks for calling it a day for me! — VH-NZZ
– VH-NZZ, Commented May 10, 2016 at 16:17

Pranav C Balan · Accepted Answer · 2016-05-10 16:24:07Z

14

The filtered array contains only one element so reduce will return that value.

Read the docs :

If the array has only one element (regardless of position) and no initialValue was provided, or if initialValue is provided but the array is empty, the solo value would be returned without calling callback.

For more : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

edited May 10, 2016 at 16:24

answered May 10, 2016 at 16:12

Pranav C Balan

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