@Gareth

when()

appears to belong to Chrome's implementation of

EventTarget

, which is common to all elements.

document.createElement("span") instanceof EventTarget
new EventTarget().when // ƒ when() { [native code] }

You can browse an object's properties and prototypes with

console.dir(...)

FWIW I don't see anything like this use of "when" on caniuse.com.