Why is Jshint saying “variable already defined” in

问题:

I have this code:

 if ( something is true ) {
        var someVar = true;
    } else {
       var someVar = false;
    }

JsHint is saying that "someVar was already defined" on the else statement part. Why is this and how do I fix it?

Thanks

回答1:

JS variables do not have block scope, they have "function" scope (or sometimes global).

The declaration (but not the assignment) is "hoisted" to the top of the function.

jshint is warning you that you have two such declarations - your code is equivalent to:

var someVar;
var someVar;  // warning!
if (something) {
     someVar = true;
} else {
     someVar = false;
}

回答2:

This is due to hoisting.

In javascript, no matter where you define a new variable with var, it moves it to the top of the function you are in. Your code is producing the following above your if block at the top of the function:

var someVar;
var someVar;

Here is a tutorial to explain hoisting:

http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-javascript-hoisting-explained/

回答3:

You shouldn't put var declarations in such places. Put the var declaration before the if, and then just set "someVar" to a value.

Indeed, here you don't need an if statement at all:

var someVar = !!(something);

will do the same thing. (The double application of ! ensures that "someVar" is set to either true or false, based on the "truthiness" of something.)