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TypeScript declaration file for function with variable number/type of arguments

headjs does some very crazy JavaScript type things to its API. For instance it takes an arbitrary number of strings (not a string array) for a function. Sometimes it ends that same function call, you can optionally end it with a function, for example.

head.js("scripturl1", "scripturl2",...,callback);

You can also (just as easily) do the following

head.js({scriptlabel:"scripturl1"},{scriptlabel2:"scripturl2"},...., callback);

My question is how the HECK do we describe that in a declaration file? I am all ears here as my current pass seems completely wrong.

Answer

The TS language spec refers to variable number/spread parameters as “Rest Parameters”. An example interface with a function signature that accepts rest params:

interface IExample {
    fn : (...args : any[]) => any;
}

var x : IExample = {
    fn: function(...args : any[]) {
        for (var i = 0, arg; arg = args[i]; i++) {
            console.log(arg);
        }
    }
}

x.fn(1);
x.fn(1, 2);
x.fn("cat", "dog", "mouse");

Unfortunately, there are some limitations. The “Rest Parameter” has to be the last one in a function’s signature — so you won’t be able to capture the type of the callback parameter since it is after the repeating parameter.

If it wasn’t, you would have been able to do something like this:

var fn = function(cb: Function, ...args : string[]) {
    ...
}