了解return语句。

Question

I have looked at many examples but still cant figure how the return statement works.

 function One() {
   var newVal = 0;
   Too();
   console.log(newVal);
 }

 function Too(newVal) {
   ++newVal; 
   return(newVal);
 }

Shouldn't this print 1 to the console? What I'm really trying to do is to have function Too increase newVal by 1 every time it's called. But I cant even figure out how to get the return statement to work. I should mention I don't want to use global variables. Thanks for any help.

Answer 1

Shouldn't this print 1 to the console?

No. The newVal inside Too is not the newVal inside One . They're completely separate.

What I'm really trying to do is to have function Too increase newVal by 1 every time it's called.

It can't, JavaScript doesn't have any mechanism for passing variables by reference like some other languages do (C#'s ref and out arguments, for instance). The closest you can come is to pass in a reference to an object and modify the object's state (which is really quite different and I won't go into it here as it would confuse things :-) ).

None of which has anything to do with the return statement.

The simplest way to do what you were describing is this:

function One() {
    var newVal = 0;
    newVal = Too(newVal);
    console.log(newVal);
}

function Too(arg) {
    ++arg; 
    return arg;
}

Here's what happens when we call One :

1. A local variable called newVal is created.
2. Its value is set to 0 .
3. A copy of its value is passed into the function Too as an argument. This has no link back to the newVal variable.
4. Too is called, accepting that value in its arg argument (arguments in JavaScript are effectively local variables).
5. Too increments the value in arg .
6. Too returns a copy of the value held by arg as its return value .
7. The return value of Too is assigned to the variable newVal .
8. We output that value (by passing it into console.log ).

Answer 2

No, it shouldn't. The return statement establishes what the value of the function invocation will be in the calling environment when control returns there. Since your calling environment doesn't use the return value, there's no net effect.

Here's how you'd get the value back:

newVal = Too(newVal);

If you want to make a function that acts as a counter, such that each time you call it you get a new number back, you can do something like this:

var counter = function(initial) {
  var c = initial || 0;
  return function() {
    return c++;
  };
}(0);

What that does is use an anonymous function to set up a persistent environment for another function, which is returned from the outer one when it's (immediately) invoked. The returned function can then be called to return a new value from its private counter (the variable "c"):

var currentCount = counter();
alert(currentCount); // 0, the first time
currentCount = counter();
alert(currentCount); // 1, and so on

Answer 3

If a function return a value you need to call that function in your console.log or capture the returned value.

function One() {
   var newVal = 0;

   console.log(Too(newVal));
 }

 function Too(newVal) {
   ++newVal; 
   return(newVal);
 }