JavaScript array reduce to sum lengths of sub-arrays - why doesn't this work?

I think this is what you're trying to do (variables renamed for clarity):

let x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];x.reduce((acc,element) => acc + element.length, 0);

This is how it would happen step-by-step:

--------------------------------------------------------------| callback | acc | element   | element.length | return value |--------------------------------------------------------------| 1st run  | 0   | [1, 2, 3] | 3              | 3            |--------------------------------------------------------------| 2nd run  | 3   | [4, 5, 6] | 3              | 6            |--------------------------------------------------------------| 3rd run  | 6   | [7, 8, 9] | 3              | 9            |--------------------------------------------------------------

The .reduce() API expects that your callback function will return the accumulator value (in your case, the value that starts off as []). Your callback just returns a value.

To make your code work, you'd need something like:

x.reduce((a,b) => { a.push(a.length + b.length); return a; }, []);

Now, if what you really want is a sum of the lengths of the arrays, then you don't want to accumulate into an array anyway; you want a simple sum. Instead of having the accumulator be an array then it just needs to be a number:

var sum = x.reduce(((sum, array) => sum + array.length), 0);

Start with zero, and each call to the function adds the array length to the sum.

As an alternative to using reduce, flatten the array and take the length of the result.

function flatten(a) { return [].concat(...a); }const x = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];const length = flatten(x).length;console.log(length);

The implementation of flatten above is for arrays which have depth of one or two. Extend/replace it as necessary for more deeply nested arrays.