Question

I’m trying to write a function
that would use its first argument, then send the rest (not known how many) to another one:

function consume_data() {
   args = consume_data.arguments;
   do_some(args[0]);
   consume_the_rest(args[1], args[2], args[3] ... );
}

Do I have to use strings to compose the call and eval or is there a neater way?

Answer 1

The traditional JavaScript way… unfortunately.

function consume_data() {

    do_some(arguments[0]);

    var args = Array.prototype.slice.call(arguments, 1);

    consume_the_rest.apply(this, args);
}

You should use the arguments variable that’s provided in the local variable scope. Then borrow .slice() from Array.prototype to get an Array of arguments starting with the second.

Then use the .apply() method to pass those arguments as a collection, which will be destructured into individual arguments in the consome_the_rest method.

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Why?

The reason you need to borrow Array.prototype.slice is that an arguments object is not really an Array, so it doesn’t have the prototyped Array methods.

The first argument given to .apply() sets the calling context (this value) of the function you’re calling, and as I mentioned above, the args Array will be destructured into individual arguments in the execution context of the function you’re calling. This is what make it possible to pass an unknown number of arguments.

---

An alternative, with caveats.

If you don’t care about the calling context of the function, you can actually do this instead…

function consume_data() {

    do_some(arguments[0]);

    consume_the_rest.call.apply(consume_the_rest, arguments);
}

Here the .call() method is just like .apply() except that you pass arguments individually instead of as a collection. So we’re calling .call() using .apply(), which will set the consume_the_rest as the calling context of .call(), and spread the arguments out, setting the first argument as the calling context arg of .call(), and the rest the normal arguments to be passed on.

It will be as though you did…

consume_the_rest.call(arguments[0], arguments[1], ...arguments[n])

…so the first argument will actually be used as the calling context, and the rest will be the ones you expect.

Again, only do this if you don’t care about the calling context of the function you’re calling.