为什么这个承诺调用之后的代码不运行？

Question

When I try to add any code after handing a function to a promise to run when it finishes, it never runs. See this example.

(function() {
   var url = //some url that I know works

   var promise = $.get(url);
   $.when(promise).then(function(xml) {
      alert("Promise resolved.");
   })();

   //This code doesn't run!
   alert("Code after promise.");
})();

I thought that execution immediately went to the next line after $.when . Isn't that the whole point of using promises? The "Promise resolved" alert goes off fine, but the "Code after promise" never appears. What am I missing?

Answer 1

You have an unnecessary () after this block:

$.when(promise).then(function(xml) {
    alert("Promise resolved.");
  })();

Change it to:

$.when(promise).then(function(xml) {
    alert("Promise resolved.");
  });

One of the comments stated that you need a $ at the beginning of your code. This is not true. You are simply creating a closure here, and this is perfectly acceptable syntax for such. I would, however, recommend prefixing your script with a ; so that if it is included after any other closures, it ensures that they are closed. This is just generally good practice when using closures in JavaScript.

Answer 2

(function(){
   var url = //some url that I know works

   var promise = $.get(url);
   //********************************
   $.when(promise).then(function(xml){
   alert("Promise resolved.");
   }); // take this () off
   //********************************
   //This code will run first
   alert("Code after promise.");
 })();