document.documentElement used in "in" statement

T.J. Crowder

That’s an expression using the in operator. The in operator checks to see if the target object has a property with the given name (directly, or via the object’s prototype).

What it’s doing is seeing if document.documentElement has a property with the name getBoundingClientRect (which is a handy function jQuery would like to use if the browser provides it).

How is it different from if(document.documentElement.getBoundingClientRect) { // do something... }

in doesn’t require fetching the value of the property, just checking that it exists. Also note that the second form you list would test the truthiness of the value of the property. In the case of a function like getBoundingClientRect that would be fine, but if you wanted to test something whose value might come back falsey (0, "", etc.), it would fail even though the property exists.

You’ll see this sort of thing a lot when doing feature-detection. For instance, if you want to know if a browser supports the HTML5 placeholder attribute:

if ('placeholder' in document.createElement('input')) {
    // Yes, it does
}

This is an example where we couldn’t use the form if (document.createElement('input').placeholder) because if the condition were false, we wouldn’t know whether it was false because the property didn’t exist, or false because the property existed but had a falsey value (and the default value for placeholder is "", which is indeed falsey).

Gratuitous link to list of feature detections.