How can I detect multidimensional array length in javascript?

Question

In the following code the matrix.length only counts the first array and not the second one. How can I count multidimensional array so the result for the following code is 10?

var matrix = [1,2,3,4,5,6, [1,2,3,4]];

console.log(matrix.length - 1)

It's unclear what you're asking. Please include the expected result in the described case and also describe the problem in general (this case is pretty specific). — hon2a
– hon2a, Commented Jun 11, 2016 at 12:25

T.J. Crowder · Accepted Answer · 2016-06-11 12:51:20Z

12

There's no shortcut, you have to sum up the lengths of the arrays. If you only have two dimensions, then:

var matrix = [1,2,3,4,5,6, [1,2,3,4]];
var sum = matrix.reduce(function(acc, entry) {
    return acc + (Array.isArray(entry) ? entry.length : 1);
}, 0);
console.log(sum);

If you may have more dimensions, you'll need recursion. As squint pointed out in a comment, making the above recursive is just a matter of giving our reduce callback a name and then doing reduce again instead of just using array.length:

var matrix = [1,2,3,4,5,6, [1,2,[3,4]]];
var sum = matrix.reduce(function callback(acc, entry) {
    return acc + (Array.isArray(entry) ? entry.reduce(callback, 0) : 1);
}, 0);
console.log(sum);

Some notes:

Array.isArray and Array#reduce were added in ES5 (in 2009) and so should be available in just about any modern browser (so, not IE8); they can both be shimmed/polyfilled on older browsers.
JavaScript doesn't actually have multi-dimensional arrays. It has arrays of arrays.
The recursive example above uses a construct called a named function expression for callback. NFEs have a bad name because years ago browsers got them wrong several different ways, but these days they're fine. (If you used the above on IE8 — including polyfills for the things it's missing — even it would work even though IE8 gets NFEs wrong.)

edited Jun 11, 2016 at 12:51

answered Jun 11, 2016 at 12:25

T.J. Crowder

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3 Comments

user1106925 Over a year ago

Just a few extra chars to make it recursive... entry.length becomes entry.reduce(fn, 0) (and fn for the callback name). Might as well go all out!

T.J. Crowder Over a year ago

@NinaScholz: Where's my head? :-) Thanks.

T.J. Crowder Over a year ago

@squint: Nice one! Done.

trincot · Accepted Answer · 2016-06-11 21:53:36Z

1

Building on @T.J. Crowder's solution with reduce, I would suggest to turn things around a bit so there is only one mention of reduce in the code. And to spice it up, this is with arrow syntax:

function entries(a) {
    return Array.isArray(a) ? a.reduce((acc, el) => acc + entries(el), 0) : 1;
}

var matrix = [1,2,3,4,5,6, [1,2, [3,4]]];

console.log(entries(matrix));

This way it will even work for non-array arguments: return value is 1 in that case. Obviously not that important.

answered Jun 11, 2016 at 21:53

trincot

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Comments

Sercan Samet Savran · Accepted Answer · 2023-06-08 07:12:21Z

1

As of 2019 the answers using reduce are deprecated (or at least there is a better solution to solve it). Make use of the Array.flat() method to "flatten" the array and take the length of it:

var matrix = [1,2,3,4,5,6, [1,2,3,4]];

// this will return you 10
console.log("Length of multidimensional array items: " matrix.flat().length)

edited Jun 8, 2023 at 7:12

answered Jun 7, 2023 at 12:14

Sercan Samet Savran

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