Question

Whenever a function is called, a special variable named arguments is added to the environment in which the function body runs. This variable refers to an object that holds all of the arguments passed to the function. Remember that in JavaScript you are allowed to pass more (or fewer) arguments to a function than the number of parameters the function itself declares.

function noArguments() {}
noArguments(1, 2, 3); // This is okay
function threeArguments(a, b, c) {}
threeArguments(); // And so is this

The arguments object has a length property that tells us the number of arguments that were really passed to the function. It also has a property for each argument, named 0, 1, 2, and so on.

If that sounds a lot like an array to you, you’re right, it is a lot like an array. But this object, unfortunately, does not have any array methods (like slice or indexOf ), so it is a little harder to use than a real array.

function argumentCounter() {
  console.log("You gave me", arguments.length, "arguments.");
}
argumentCounter("Straw man", "Tautology", "Ad hominem");
// → You gave me 3 arguments.

Some functions can take any number of arguments, like console.log . These typically loop over the values in their arguments object. They can be used to create very pleasant interfaces. For example, remember how we created the entries to Jacques’ journal.

addEntry(["work", "touched tree", "pizza", "running",
          "television"], false);

Since he is going to be calling this function a lot, we could create an alternative that is easier to call.

function addEntry(squirrel) {
  var entry = {events: [], squirrel: squirrel};
  for (var i = 1; i < arguments.length; i++)
    entry.events.push(arguments[i]);
  journal.push(entry);
}
addEntry(true, "work", "touched tree", "pizza",
         "running", "television");

This version reads its first argument ( squirrel ) in the normal way and then goes over the rest of the arguments (the loop starts at index 1, skipping the first) to gather them into an array.

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.