Question

Is this legal? And does it work in all browsers?

function func1(a, b, c) {
  //b == 2 here
  change_em(arguments);
  //b should equal 3 here
}

function change_em(args) {
  args[0] = 6;
  args[1]++;
  args[2] = [];
}

func1('foo', 2);

If you are wondering, I need to adjust the arguments in a function. A macro would be perfect, except javascript doesn’t have one. Doing it properly, i.e. passing the variables to a function, then returning them in an object, and then extracting them in the caller would be almost as much code as just copy/pasting the adjuster function.

Answer 1

It is valid under EcmaScript 3 and EcmaScript 5 non-strict, but it is not valid under EcmaScript 5 strict mode. It works in all modern browsers and most old ones. It should not work in strict mode such as when run in Firefox 5 with a "use strict" directive.

From the EcmaScript 5 spec section 10.6.11.c.ii

10.6 Arguments Object

…

If strict is false and name is not an element of mappedNames, then

1. Add name as an element of the list mappedNames.

2. Let g be the result of calling the MakeArgGetter abstract operation with arguments
   name and env.

3. Let p be the result of calling the MakeArgSetter abstract operation with arguments
   name and env.

4. Call the [[DefineOwnProperty]] internal method of map passing ToString(indx), the
   Property Descriptor {[[Set]]: p, [[Get]]: g, [[Configurable]]: true}, and false as
   arguments.

Basically, the arguments object gets a setter for each index so that assigning to arguments[i] changes the value of the named parameter at position i. There is language in the spec which makes it work vice-versa.

This should work in non-strict mode in any interpreter

(function (x) {
  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // Both should be 0
  arguments[0] = 1;
  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // both should be 1
  x = 2;
  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // both should be 2
 })(0);

But if you run the above on Firefox 5 with a use strict directive you get different behavior:

(function (x) {
  "use strict";

  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // Both should be 0
  arguments[0] = 1;
  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // x=0, arguments[0]=1
  x = 2;
  alert("x=" + x + ", arguments[0]=" + arguments[0]);  // x=2, arguments[0]=1 
 })(0);