使用函数或类型

Question

How much slower faster is the typeof operator than a function call? Or is it negligible and micro-optimising?

if (isNumber(myVar)) {
}

if (typeof myVar === 'number') {
}

Answer 1

Or is it negligible and micro-optimising?

Yes, this is definitely something to worry about if and only if you identify the code in question as being a performance bottleneck, which is really unlikely. It's micro-optimization. Function calls are really, really fast even if they don't get optimized out by the JavaScript engine. I used to worry about function call overhead when Array#forEach first appeared on the scene. Even back then , it wasn't an issue, even on the oldest, slowest JavaScript interpreter I could find: The one in IE6. Details on my blog: foreach and runtime cost

Re whether it takes longer... How long is a piece of string? It totally depends on the JavaScript engine you're using and whether the code in question is identified as a "hot" spot by the engine (assuming it's an engine like V8 that works in stages and optimizes hot spots).

A modern engine is likely to inline that if it becomes important to do so. That is not a guarantee.

Answer 2

Or is it negligible and micro-optimising?

It's negligible and micro-optimizing.

---

If you want to check if something's a number, I recommend using an isNaN check and then casting to a number.

if (!isNaN(myVar)) {
  myVar = +myVar;
}

In this way, you don't actually care how the value gets treated as a number.

Someone using the API could then choose to pass an object that can be treated as a number:

myVar = {
  valueOf: function () {
    return 5;
  }
};