Javascript范围变量（全局/本地）

Question

I want to use a local variable as a global variable and I was told the way to do so is to create the variable outside the function, like this:

        var foo = null;

        function bar() {
            foo = 3;
        }

        console.log(foo);

However the foo logs null, instead of 3. This is the simplest example I could think of, but mainly I want to be able to use the data from a variable I created in a function throughout my whole script.

What am I doing wrong?

---

Same question in an Ajax request :

var status = null;

.ajax({
  url: myUrl, 
  success: function (data) {
    status = data.status; //returns a string
    console.log(status); // it works, I get the string
  }, 
  dataType: 'jsonp' 
});

console.log(status); // still null

Answer 1

You need to call the function first like this.

    var foo = null;

    function bar() {
        foo = 3;
    }

    bar();
    console.log(foo);

---

Your best bet here is to call a function that logs the data after the ajax runs. In this example I created a function called logit() which is called in the ajax request state.

var status = null;

$.ajax({
  url: myUrl, 
  success: function (data) {
    status = data.status; //returns a string
    console.log(status); // it works, I get the string
    logit();
  }, 
  dataType: 'jsonp' 
});

function logit(){
    console.log("logit(): " + status);
}

The reason you are getting null is because this is the order that your code is running:

1. sets status to null
2. saves the ajax to memory to run when the requests completes this doesn't actually run yet
3. logs status which is still null because the success callback hasn't run yet
4. the ajax request finally completes and runs the success callback
5. status is set to data.status