All objects in JavaScript are truthy per the spec, but in the DOM one non-primitive object is not. Which?

dom, javascript, object

Solution

Disclaimer: I’m the guy who tweeted that :) It was a question I would ask and answer in my Front-Trends talk. I wrote that tweet 5 minutes before going on stage.

Because of the 140-character limit on Twitter, the question is slightly ambiguous. The real question I was asking is the following.

The ECMAScript spec defines `ToBoolean()` as follows:

As you can see, all non-primitive objects (i.e. all objects that aren’t a boolean, a number, a string, `undefined`, or `null`) are truthy as per the spec. However, in the DOM, there is one exception to this — a DOM object that is falsy. Do you know which one that is? The answer is `document.all`. The HTML spec says:

The `all` attribute must return an `HTMLAllCollection` rooted at the `Document` node, whose filter matches all elements.

The object returned for all has several unusual behaviors:

The user agent must act as if the `ToBoolean()` operator in JavaScript converts the object returned for `all` to the `false` value.

The user agent must act as if, for the purposes of the `==` and `!=` operators in JavaScript, the object returned for `all` is equal to the `undefined` value.

The user agent must act such that the `typeof` operator in JavaScript returns the string `'undefined'` when applied to the object returned for `all`.

These requirements are a willful violation of the JavaScript specification current at the time of writing (ECMAScript edition 5). The JavaScript specification requires that the `ToBoolean()` operator convert all objects to the `true` value, and does not have provisions for objects acting as if they were `undefined` for the purposes of certain operators. This violation is motivated by a desire for compatibility with two classes of legacy content: one that uses the presence of `document.all` as a way to detect legacy user agents, and one that only supports those legacy user agents and uses the `document.all` object without testing for its presence first.

So, `document.all` is the only official exception to this ECMAScript rule. (In Opera, `document.attachEvent` etc. are falsy too, but that’s not specced anywhere.)

Problem

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