Javascript（变量范围）：变量未获取分配给它的值

Question

I'm very new to JS and I'm trying to set the value of a variable within a function but it is 'undefined' after setting the value. My code is as follows:

    (function (name, context, definition){ 'use strict' ...} ('FP2', this, function (){...})
var fp = new FP2();   
fp.get(function(result, components) { for (var index in components){...};
    var IP = 'nothing';                                                               
    $.getJSON('http://gd.geobytes.com/GetCityDetails?callback=?', function(data) {
      this.IP = JSON.stringify(data, null, 2);
      alert(this.IP);
    });

    alert(IP);
});

I expect that the IP variable have the same value inside and outside the function, but it does not. The inner alert shows the expected value but the alert outside the function shows 'nothing'. From similar questions 1 , 2 , 3 , I'm not using var to redeclare a local variable inside the function, and I'm pointing to the IP variable outside the function using 'this.', so I don't understand why it does not work as expected?

Answer 1

No need to use this.IP , when you use this inside the API call, you are referring to that API call function itself. Just using IP should work.

The alert(IP); outside the API call wouldn't return the result as well. This function call will be immediately called even before the API returns some data so it will print out the previous value undefined .

(function (name, context, definition){ 'use strict' ...} ('FP2', this, function (){...})
var fp = new FP2();   
fp.get(function(result, components) { for (var index in components){...};
    var IP = undefined;                                                               
    $.getJSON('http://gd.geobytes.com/GetCityDetails?callback=?', function(data) {
     IP = JSON.stringify(data, null, 2);
     alert(IP);
    });

    alert(IP); //This will return undefined
});

Answer 2

If you want get IP value at anywhere, use follow method:

var IP = undefined;
var fpData = {};
function setIP(){
  var valueIP = undefined;
  $.ajax({
        url: "http://gd.geobytes.com/GetCityDetails?callback=?",
        async: false,
        dataType: 'json',
        success: function(data) {
            valueIP = JSON.stringify(data, null, 2); //console.log(valueIP);
            //return valueIP;
            localStorage.setItem('IP', valueIP);
        }
    });
  //return valueIP;
}

//call setIP
setIP();
IP = localStorage.getItem('IP');
console.log(IP);