javascript - global variable not in scope after being defined in callback?

问题

whenever i try to run something like the following, firebug tells me that "markers is undefined" at the line "for (var i=0 ..."

but i declared markers as a global variable at the top right...?

var markers;
function load() {

  $.get("phpsqlajax_genxml.php", function(data) {
    markers = data.documentElement.getElementsByTagName("marker");
  });

  for (var i = 0; i < markers.length; i++) {
    var name = markers[i].getAttribute("name")
    //do more stuff
    }
}

but when i do this, it works.

var markers;
function load() {

  $.get("phpsqlajax_genxml.php", function(data) {
      markers = data.documentElement.getElementsByTagName("marker");
      makeMarkersWithXMLinfo(); 
  });

  function makeMarkersWithXMLinfo() {
      for (var i = 0; i < markers.length; i++) {
             var name = markers[i].getAttribute("name")
             //do more stuff
      }
  }
}

i'm not even passing "markers" as an argument to my makeMarkersWithXMLinfo() function. but yet it works. what's going on? thnx

回答1:

The problem you're having is that get starts an asynchronous operation. So your code immediately following the call to get happens before the success callback on the get is run. E.g. (see the comments):

var markers;
function load() {

  // ===> This happens FIRST
  $.get("phpsqlajax_genxml.php", function(data) {
    // ===> This happens THIRD, some time after `load` returns
    markers = data.documentElement.getElementsByTagName("marker");
  });

  // ===> This happens SECOND
  for (var i = 0; i < markers.length; i++) {
    var name = markers[i].getAttribute("name")
    //do more stuff
    }
}

Your second example is the correct way to code it (although I'd recommend avoiding a global entirely), because you're using markers only after the GET has completed.

回答2:

$.get is asynchronous, that means that if you call something immediatly after $.get, it's callback function wouldn't be invoked yet, and your global would still be undefined.

来源：https://stackoverflow.com/questions/8960803/javascript-global-variable-not-in-scope-after-being-defined-in-callback

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javascript

variables

loops

callback

scope