Question

Content negotiation

See the various examples of using an XMLHttpRequest in this chapter for an example of the method calls involved in making a request. You can use a synchronous request (by setting the third parameter to open to false ) if you want.

Asking for a bogus media type will return a response with code 406, “Not acceptable”, which is the code a server should return when it can’t fulfill the Accept header.

Waiting for multiple promises

The function passed to the Promise constructor will have to call then on each of the promises in the given array. When one of them succeeds, two things need to happen. The resulting value needs to be stored in the correct position of a result array, and we must check whether this was the last pending promise and finish our own promise if it was.

The latter can be done with a counter, which is initialized to the length of the input array and from which we subtract 1 every time a promise succeeds. When it reaches 0, we are done. Make sure you take the situation where the input array is empty (and thus no promise will ever resolve) into account.

Handling failure requires some thought but turns out to be extremely simple. Just pass the failure function of the wrapping promise to each of the promises in the array so that a failure in one of them triggers the failure of the whole wrapper.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.