How to assign a chrome tab to a variable by a given tab id in javascript?

I tried to assign a tab to a variable; but I failed. Here is my code:

var tab;
chrome.tabs.get开发者_如何转开发(id, function(t) {
  tab = t;
  console.log("tab: " + tab); // here the printed result is right
});
console.log("(after assignment) tab: " + tab); // always get "undefined"

The code is simple but I don't know where the problem is...

---

The problem is in asynchronous Chrome API calls, meaning that you don't receive response from them immediately. You can rewrite your code in this way for example:

var tab;
chrome.tabs.get(id, function(t) {
  tab = t;
  console.log("tab: " + tab); // here the printed result is right

  afterAssignment();
});

function afterAssignment() {
    console.log("(after assignment) tab: " + tab); // always get "undefined"

    //the rest of the code that needs to use tab var
}