typeof是一个操作符和一个函数

Question

In JavaScript typeof is an operator and a function. Is it better used as an operator or a function? Why?

Answer 1

typeof is an operator. You can easily check it using:

typeof(typeof)

Were typeof a function, this expression would return 'function' string, but it results in a syntax error:

js> typeof(typeof);
typein:8: SyntaxError: syntax error:
typein:8: typeof(typeof);
typein:8: .............^

so, typeof cannot be a function. Probably parenthesis-notation typeof(foo) made you think typeof is a function, but syntactically, those parenthesis are not function call - they are those used for grouping, just like (2 + 3) *2 . In fact, you can add any number of them you want:

typeof(((((foo))))); // is equal to typeof foo;

Answer 2

I think you pick which you want based on clarity, as a habit I usually use it as an operator in the following way because it's pretty clear, at least IMO:

if(typeof thing === "string") { 
  alert("this is a string");
}

if(typeof thing === "function") {
  alert("this is a function");
}

This is opposed to this format:

if(typeof(thing) === "string") { 
  alert("this is a string");
}

Which to me, makes it slightly slower to read. If you do typeof(thing) it's the same thing, so whatever floats your boat. You can get a full read and what strings to expect from types here .