Question

I am using an object as a hash table and I have stuffed both regular properties and integers as keys into it.

I am now interested in counting the number of keys in this object which are numbers, though obviously a for (x in obj) { if (typeof x === "number") { ... } } will not produce the result I want because all keys are strings.

Therefore I determined that it is sufficient for my purposes to assume that if a key’s first character is a number then it must be a number so I am not concerned if key “3a” is “wrongly” determined to be a number.

Given this relaxation I think i can just check it like this

for (x in obj) {
  var charCode = x.charCodeAt(0);
  if (charCode < 58 && charCode > 47) { // ascii digits check
     ...
  }
}

thereby avoiding a regex and parseInt and such.

Will this work? charCodeAt is JS 1.2 so this should be bullet-proof, yes?

Hint: I would love to see a jsperf comparing my function with what everyone comes up with. 🙂 I’d do it myself but jsperf confuses me

Update: Thanks for starting up the JSPerf, it confirms my hope that the charCodeAt function would be executing a very quick piece of code reading out the int value of a character. The other approaches involve parsing.

Answer 1

Why speculate when you can measure. On Chrome, your method appears to be the fastest. The proposed alternatives all come at about 60% behind on my test runs.