After calling chrome.tabs.query, the results are not available

Question

I'm creating (learning) an extension for Google Chrome.

To debug some code, I inserted console.log(), as follows:

var fourmTabs = new Array(); chrome.tabs.query({}, function (tabs) {     for (var i = 0; i < tabs.length; i++) {         fourmTabs[i] = tabs[i];     } }); for (var i = 0; i < fourmTabs.length; i++) {     if (fourmTabs[i] != null)         window.console.log(fourmTabs[i].url);     else {         window.console.log("??" + i);     } } 

It's very simple code: get all tabs info into an array of my own, and print some things.

To check whether the code works as it should, I run the code. Here comes the problem:

• When I use breakpoints (via the Developer tools), the code runs fine.
• Without breakpoints, nothing is printed.

Any idea why?

Answer 1

Your problem can be simplified to:

/*1.*/ var fourmTabs = []; /*2.*/ chrome.tabs.query({}, function(tabs) { /*3.*/     fourmTabs[0] = tabs[0]; /*4.*/ }); /*5.*/ console.log(fourmTabs[0]); 

You expect that the fourmTabs array is updated (by line 3) when line 5 is reached.
That is wrong, because the chrome.tabs.query method is asynchronous.

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In an attempt to make you understand the significance of the asynchronous aspect, I show a code snippet with the same structure as your code and a story.

/*1.*/ var rope = null; /*2.*/ requestRope(function(receivedRope) { /*3.*/     rope = receivedRope; /*4.*/ }); /*5.*/ grab(rope); 

• At line 1, the presence of a rope is announced.
• At lines 2-4, a callback function is created, which ought to be called by the requestRope function.
• At line 5, you're going to grab the rope via the grab function.

When requestRope is implemented synchronously, there's no problem:
  You: "Hi, I want a rope. Please throw the rope^{"call the callback function"} when you've got one."
  She: "Sure." throws rope
  You: Jumps and grabs rope - You manage to get at the other side, alive.

When requestRope is implemented asynchronously, you may have a problem if you treat it as synchronous:
  You: "Please throw a rope at me."
  She: "Sure. Let's have a look..."
  You: Jumps and attempts to grab rope Because there's no rope, you fall and die.
  She: Throws rope _{Too late, of course.}

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Now you've seen the difference between an asynchronously and synchronously implemented function, let's solve your original question:

var fourmTabs = new Array(); chrome.tabs.query({}, function (tabs) {     for (var i = 0; i < tabs.length; i++) {         fourmTabs[i] = tabs[i];     }     // Moved code inside the callback handler     for (var i = 0; i < fourmTabs.length; i++) {         if (fourmTabs[i] != null)            window.console.log(fourmTabs[i].url);         else {             window.console.log("??" + i);         }     } }); // <moved code inside callback function of chrome.tabs.query> 

With breakpoints, your code works, because by the time that the second part of the code is reached, the callback has already been called.