Javascript Quiz: Question 1

Question: What is the output?

var o = {
  a: 1,
  b: 2,
  c: 3
};
for (var x in o) {
  setTimeout(function() {
    console.log(x);
  }, 1000 * Math.random());
}

The Math.random() is there to trick you.

c
c
c

for (var x in o) {
  (function(y) {
    setTimeout(function() {
      console.log(y);
    }, 1000 * Math.random());
  }).call(this, x);
}

The Dojo Toolkit has a d.forEach method which eliminates closure issues like this, but is designed to work on array or NodeList objects. I am not aware of any dojo methods for iterating over object properties.

For closures sake I put together a library method that no doubt is already in dojo somewhere. Thanks to the JSLint error, "The body of a for in should be wrapped in an if statement to filter unwanted properties from the prototype" I was able to research the reasoning behind the warning and come up with this forEach implementation:

function forEach(o, context, f) {
  var x;
  if (!f) {
    f = context;
    context = this;
  }
  for (x in o) {
    if (o.hasOwnProperty(x)) {
      f.call(context, x, o[x], o);
    }
  }
}

// "abc: 6"
(function() {
  var x = 0,
      $x = "";
  forEach(o, function(name, value) {
    $x += name;
    x += value;
  });
  alert($x + ": " + x);
}()); 

References
http://stackoverflow.com/questions/1963102/what-does-the-jslint-error-body-of-a-for-in-should-be-wrapped-in-an-if-statement
http://www.jslint.com/
http://www.google.com/search?q=js+closures