修改javascript中的局部变量

Question

the descriptionPercentagesStr variable is not modified in the success queryExt.first.

I think it is a syntax error but can not find.

   var ProductExtendidoClass = Parse.Object.extend("Product");
    var productExtObj = new ProductExtendidoClass();
    var idProduct =this.model.attributes.id;
    var queryExt = new Parse.Query(ProductExtendidoClass);

    descriptionPercentagesStr="inicial";


    queryExt.equalTo("idProduct", idProduct);

    queryExt.first({
      success: function(productExtObj) {

        if (productExtObj!=null)// se tiene el producto
        {
        );
           $("input[name=description_percentages]").val(productExtObj.get("descriptionPercentages"));
           descriptionPercentagesStr=productExtObj.get("descriptionPercentages");

        }
        else
        {
          //alert("lengt es menor o igual a 0");

        }
        // Successfully retrieved the object.
      },
      error: function(error) {
        alert("Error: " + error.code + " " + error.message);
      }
    });
  alert(descriptionPercentagesStr);

Answer 1

Note that the first query function is asynchronous. The success and failure functions are its callbacks and are called at a later arbitrary time when the query completes. They are not called sequentially as with the rest of your code. So the order of operations in your code is as follows:

1. Set descriptionPercentagesStr to "inicial"
2. Query the ProductExtendidoClass for the matching idProduct (results not received yet)
3. Alert descriptionPercentagesStr (returns "inicial")
4. Query completes at some later time and descriptionPercentagesStr is set to the result

You can verify this behavior by adding alert(descriptionPercentagesStr); into the success callback function. It should output the result from Parse.