JavaScript性能（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

Answer 1

The way to answer performance questions is to run benchmarks.

There are four alternatives:

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

2. typeof arr[i] === 'undefined' || num < arr[i]

3. !(i in arr) || 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 .

4. !(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 .

Answer 2

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;

Answer 3

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