Javascript中的“：”运算符

Question

I'm using project polymer https://www.polymer-project.org/ and trying to precompile some Webcomponents in a Rails app.

I get an error in the file:

https://polymer-topeka.appspot.com/components/firebase/firebase.js

So I guess it is malformed.

After I decompressed it with http://jsbeautifier.org/ I tried to use jstocoffee to check what could be a syntax error:

http://js2coffee.org/

And found this:

function Ua(a, b) {
  return a > b ? 1 : a < b ? -1 : 0
}
var Va;
a: {
  var Wa = aa.navigator;
  if (Wa) {
    var Xa = Wa.userAgent;
    if (Xa) {
        Va = Xa;
        break a
    }
  }
  Va = ""
}

What does the "a:" mean? Because js2coffee sends an error right there. It is a valid operator?

Answer 1

The a: in your code is a label. JavaScript labels work with break and continue statements as a (somewhat limited) form of "go to".

Thus this code:

a: {
  var Wa = aa.navigator;
  if (Wa) {
    var Xa = Wa.userAgent;
    if (Xa) {
        Va = Xa;
        break a
    }
  }
  Va = ""
}

works as if it had been written:

var var Wa = aa.navigator;
if (Wa && (Xa = Wa.userAgent) {
  Va = Xa;
}
else {
 Va = "";
}

So Va gets set to the empty string if the code can't find both the navigator and navigator.userAgent values.

Answer 2

In the function

function Ua(a, b) { 
    return a > b ? 1 : a < b ? -1 : 0
}

Says: if a is greater than b then return 1; else if a is less than b return -1; else return 0.

the other a statement

a: {

is a global variable. It doesn't have anything to do with the function.