Count of “Defined” Array Elements

问题

Given following array:

var arr = [undefined, undefined, 2, 5, undefined, undefined];

I'd like to get the count of elements which are defined (i.e.: those which are not undefined). Other than looping through the array, is there a good way to do this?

回答1:

In recent browser, you can use filter

var size = arr.filter(function(value) { return value !== undefined }).length;

console.log(size);

Another method, if the browser supports indexOf for arrays:

var size = arr.slice(0).sort().indexOf(undefined);

If for absurd you have one-digit-only elements in the array, you could use that dirty trick:

console.log(arr.join("").length);

There are several methods you can use, but at the end we have to see if it's really worthy doing these instead of a loop.

回答2:

An array length is not the number of elements in a array, it is the highest index + 1. length property will report correct element count only if there are valid elements in consecutive indices.

var a = [];
a[23] = 'foo';
a.length;  // 24

Saying that, there is no way to exclude undefined elements from count without using any form of a loop.

回答3:

No, the only way to know how many elements are not undefined is to loop through and count them. That doesn't mean you have to write the loop, though, just that something, somewhere has to do it. (See #3 below for why I added that caveat.)

How you loop through and count them is up to you. There are lots of ways:

A standard for loop from 0 to arr.length - 1 (inclusive).
A for..in loop provided you take correct safeguards.
Any of several of the new array features from ECMAScript5 (provided you're using a JavaScript engine that supports them, or you've included an ES5 shim, as they're all shim-able), like some, filter, or reduce, passing in an appropriate function. This is handy not only because you don't have to explicitly write the loop, but because using these features gives the JavaScript engine the opportunity to optimize the loop it does internally in various ways. (Whether it actually does will vary on the engine.)

...but it all amounts to looping, either explicitly or (in the case of the new array features) implicitly.

回答4:

Loop and count in all browsers:

var cnt = 0;
for (var i = 0; i < arr.length; i++) {
    if (arr[i] !== undefined) {
        ++cnt;
    }
}

In modern browsers:

var cnt = 0;
arr.foreach(function(val) {
    if (val !== undefined) { ++cnt; }
})

回答5:

Unfortunately, No. You will you have to go through a loop and count them.

~~EDIT :~~

var arrLength = arr.filter(Number);
alert(arrLength);

回答6:

If the undefined's are implicit then you can do:

var len = 0;
for (var i in arr) { len++ };

undefined's are implicit if you don't set them explicitly

//both are a[0] and a[3] are explicit undefined
var arr = [undefined, 1, 2, undefined];

arr[6] = 3;
//now arr[4] and arr[5] are implicit undefined

delete arr[1]
//now arr[1] is implicit undefined

arr[2] = undefined
//now arr[2] is explicit undefined

回答7:

Remove the values then check (remove null check here if you want)

const x = A.filter(item => item !== undefined || item !== null).length

With Lodash

const x = _.size(_.filter(A, item => !_.isNil(item)))

来源：https://stackoverflow.com/questions/9901950/count-of-defined-array-elements

标签

javascript

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