Question

The title may not make sense and nor would a description alone, so here’s a sample of the code:

for(var a=0;a<10;a++) {
    var b=document.createElement('b')
    b.onclick=function() {
        alert(a)
    }
    b.innerHTML=a
    document.body.appendChild(b)
}

What’s odd is that when I set innerHTML to a, it uses the current value of a. So, this code creates ten <b> elements with the values 0, 1, 2, 3, 4, 5, 6, 7, 8, & 9. This works perfectly. What doesn’t, however, is the onclick function. It returns the final value of a (10) when any of these elements is clicked. I’ve tried setting another variable to a and then using that other variable in the onclick event, but still it returns the final value of a. How would I make it use the value of a when onclick is set?

Answer 1

Try the following

b.onclick= function(arg) {
    return function() {
        alert(arg);
    }
}(a);

The problem you were hitting is that javascript doesn’t use block scope. Instead all variables have a lifetime of their containing function. So there was only ever one a captured in all of the onclick functions and they all see it’s final value.

The solution works around this by creating a new function and value per scope and then immediately setting that value to the current value of a.

Here’s a version that’s a bit expanded and perhaps a bit more readable

var createClickHandler = function(arg) {
  return function() { alert(arg); };
}

for(var a=0;a<10;a++) {
    var b=document.createElement('b')
    b.onclick = createClickHandler(a);
    b.innerHTML=a
    document.body.appendChild(b)
}