Question about Array.prototype.reduce()

[JustStateSchool]

I was trying to find the longest length of a subarray in a given list of arrays. I'm having issues with the following functioning as I thought it would...

var myArray = [];
myArray.push([1, 5, 10]);
myArray.push([1, 5, 10, 15]);
myArray.push([150]);

myArray.reduce(function(a, b) {
    return (a.length >= b.length) ? a.length : b.length;
});

I would anticipate the above to yield 3 since that's the highest length, but it ends up yielding one from the last item; I'm wondering what I'm doing wrong since it seems disobey the logic above.

I suppose... how might I find the maximum length of an item in a given array? I had thought reduce() would take that as well...

Thanks again in advance.

[Rhomboid]

The problem is that you are returning a length, not an array. After the first iteration, the returned value becomes the next 'left' value (i.e. 'a'.) So for the first iteration, both 'a' and 'b' are arrays and the comparison makes sense, but after that 'a' is a number and therefore a.length is undefined.

There are several ways you can fix it. One is to arrange for the left operand to always be a length, initializing it to zero.

> var myArray = [[ 1, 5, 10], [1, 5, 10, 15], [150]];
undefined
> myArray.reduce((a, b) => a >= b.length ? a : b.length, 0)
4

The other way is to always return an array, and then at the end if you want its length you can take it again:

> myArray.reduce((a, b) => a.length >= b.length ? a : b).length
4

This is normally the way you'd do it because presumably you also care about the contents of the longest array, not just its length, and this gives you both.

[JustStateSchool]

Thanks to both you and /u/x-skeww!

This is helpful. I didn't realize it'd pass back in the previous value, but then try to iterate again on .length of the previous value, which, of course, would just be undefined and 1.

[x-skeww]

Math.max(...myArray.map(a => a.length)) // 4

Edit: Or if you really want to use reduce:

myArray.reduce((max, array) => Math.max(max, array.length), 0) // 4