Question

Why does Modernizr do the following:

toString = {}.toString,

Answer 1

It’s grabbing a local copy of the Object.prototype.toString method which would allow it to make small speed improvements in the script. This also allows it to test that the toString method exists.

Regards to comments:

Every name resolution there is a cost, in lookup-time (locals, globals, prototype-chaining) and creation (closure-scoped variable), so imaging the following code:

var values = // Create some object here.

for (var i = 0; i < count; i++) {
  console.log(values[i].toString());
}

For each iteration of the look we are having to resolve the values variable, and walk the prototype chain to identify the member toString, and then execute that.

Taking that above example, we could do the following:

var toString = {}.toString,
    values = // Create some object here.

for (var i = 0; i < count; i++) {
  console.log(toString.call(values[i]));
}

Or even further:

var toString = {}.toString,
    log = console.log,
    values = // Create some object here.

for (var i = 0; i < count; i++) {
  log.call(console, toString.call(values[i]));
}

Light applications won’t really benefit too much from this, but larger frameworks, such as jQuery, etc, can improve the script performance quite significantly. IE I believe is one such browser where these small improvements can help quite a lot.