JavaScript Performance (typeof arr[i] === "undefined" || num < arr[i])

Question

I'm looking for a way to avoid the typeof check on every arr[i]

I need to accommodate the fact that arr[i] can be 0

I want to assign arr[i] = num if arr[i] is undefined or greater than num

Why would it be `undefined` in the first place? Doesn't seem like a useful value to have in an Array of numbers. Either way, there's no need for `typeof` to check for `undefined`. — , Dec 25 '15 at 03:03
Why would you need to make such simple logic faster in the first place? It ain't gonna get much faster than that. You can try something like `if(!(num >= arr[i])){arr[i]=num;}` though to invert what happens for the case of it being undefined. — Ultimater, Dec 25 '15 at 07:53

score 2 · Answer 1 · 2015-12-25T07:30:23.133

The way to answer performance questions is to run benchmarks.

There are four alternatives:

arr[i] === undefined || num < arr[i]
typeof arr[i] === 'undefined' || num < arr[i]
!(i in arr) || num < arr[i]. This assumes the undefined values are true "holes" (missing indexes), rather than indexes that are present but have a value of undefined.
!(num >= arr[i]). This takes advantage of the fact that comparisons with undefined return false.

1 and 2 are equivalent performance-wise. 3 is about 20 times slower. 4 is about 20% slower. Using isNaN as suggested in another answer is about 50% slower.

Therefore, if you are interested purely in performance, use 1 or 2. If you want to save keystrokes, at a minor performance cost, use 4. However, 4 is also less readable and you probably will have to add a comment about how it handles undefined, which will negate any keystroke savings.

See http://jsperf.com/ways-to-check-for-undefined/4.

score 1 · Answer 2 · edited Mar 12 '16 at 02:25

1

Try this:

arr[i] == undefined || num < arr[i]

or

arr[i]=(arr[i] >= 0 && arr [i] < num) ? arr[i] : num;

Or

arr[i]=(!isNaN(arr[i]) && arr [i] < num) ? arr[i] : num;

edited Mar 12 '16 at 02:25

hichris123

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answered Dec 25 '15 at 02:14

Manivannan

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This will replace negative values of `arr[i]` with `num`, which probably is not what the OP intended. – Dec 25 '15 at 06:12
For this case you can try with isNaN method – Manivannan Dec 25 '15 at 07:04
2

`isNaN` seems to be about 50% slower than a direct check for `undefined`. See http://jsperf.com/ways-to-check-for-undefined/5. – Dec 25 '15 at 07:28
Oh thanks for update.. For this case, Your 1st option is cool but my suggetion is not to check type `arr[i] == undefined || num < arr[i]` – Manivannan Dec 25 '15 at 13:46

score 1 · Answer 3 · edited May 23 '17 at 12:14

Checking something === something_else is as fast as it gets, but what does make your code slower is the fact that you are doing two things instead of one:

You are checking both typeof arr[i] === “undefined” and num < arr[i] even though javascript engine is going to check data type anyway.

What I mean is, if it is true that typeof arr[i] === “undefined”, then num < arr[i] will always be false. It would also be false for >=, which leads to the solution.

Instead of:

if (typeof arr[i] === “undefined” || num < arr[i])

You can do:

if (!(num >= arr[i]))

On the other hand...

if you don't know that it is currently too slow, which you don't if you did not profile it, then you should not optimize it. See: When is optimisation premature?
why do you even have undefined data in the list to begin with? Couldn't you filter that out before writing into the list, rather than at reading time? It is a good idea to always have only one data type in a contaioner (unless you have a really good reason otherwise)
if you have a key->value mapping with some keys not defined, maybe Array is not the right container for your use case

JavaScript Performance (typeof arr[i] === "undefined" || num < arr[i])

3 Answers3

On the other hand...