Etiqueta: arguments

Evil side effects of changing arguments

I’ve just realized about a creepy fact about JavaScript. Suppose you have this code:

function f(a, b, c) {
  arguments[1] = 99;
  return [a, b, c];
}
f(1, 2, 3);

What do you think the result is?

[1, 2, 3]

You guess but NO. Right answer is:

[1, 99, 3]

Unless you’re in strict mode (and you should always be in strict mode), then result is [1, 2, 3].

In non strict mode, setting the n-th item of arguments alters the value of the n-th formal parameter in the function signature. A really evil and unexpected side effect but specified in http://ecma-international.org/ecma-262/5.1/#sec-10.6 (see Note 1).

So be careful and keep yourself in strict mode where things are less unpredictable!

function f(a, b, c) {
  'use strict';
  arguments[1] = 99;
  return [a, b, c];
}
f(1, 2, 3);