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Invalid javascript array element is undefined

I create an array with let arr = new Array(99999)

, but I do not fill it up to arr.length

, which is 99999

, how can I know how many actual, non- undefined

elements I have in this array?

Is there a better way than looking for the first one undefined

?

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2 answers


You can use Array#forEach

that skips sparse elements.



let array = new Array(99999),
    count = 0;

array[30] = undefined;

array.forEach(_ => count++);

console.log(count);
Run codeHide result


The same with Array#reduce 



let array = new Array(99999),
    count = 0;

array[30] = undefined;

count = array.reduce(c => c + 1, 0);

console.log(count);
Run codeHide result


To filter out unsharp / dense elements, you can use a callback that returns for each element true

.

Perhaps this link helps a little to understand the mechanics of sparse array: JavaScript: sparse arrays and dense arrays .



let array = new Array(99999),
    nonsparsed;

array[30] = undefined;

nonsparsed = array.filter(_ => true);

console.log(nonsparsed);
console.log(nonsparsed.length);
Run codeHide result


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The fastest and easiest way to filter elements in an array is ... well ... use a function to .filter()

filter out only those elements that are valid (not undefined in your case) and then check the .length

result ...



function isValid(value) {
  return value != undefined;
}
var arr = [12, undefined, "blabla", ,true, 44];
var filtered = arr.filter(isValid);

console.log(filtered);   // [12, "blabla", true, 44]
Run codeHide result


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