Named arguments and the arguments object in JavaScript

Question

Recently I started learning JavaScript through Nicholas C. Zakas' book Professional JavaScript For Web Developers and I came across some questions that I could not solve by myself. As the title says, that's all about named arguments and arguments object in JavaScript functions. E.g. we have this piece of code: ``` function doAdd(num1 , num2) { arguments[1] = 10; alert(arguments[0] + num2); } doAdd(10 , 20); ``` The book says that values in the arguments object are automatically reflected by the corresponding named arguments, so `num2` enters the function with a value of 20 and then it gets overwritten via `arguments[1]` and finally gets a value of 10.All good all clear up to this point. Then it says that this effect goes only one way: changing the named argument does not result in a change to the corresponding value in arguments. And that's where the problems start. I tried to modify this code a bit to understand what the author says but I failed. E.g. ``` function doAdd(num1 , num2)…

Accepted Answer

This is because the book assumes you're using strict mode.

If you run the code in strict mode, you'll get the behavior you'd expect given what's written in the book.

```
function doAdd(num1 , num2) {
     "use strict";
     arguments[1] = 10;
     num2 = 40;
     alert(arguments[0] + arguments[1]);
}
doAdd(10 , 20); // this alerts 20, alerts 50 in nonstrict mode
```

A little known fact is that the entire point of strict mode is to avoid this sort of dynamic scoping (which is why arguments is fixated, `with` isn't allowed and `eval` behaves differently). Aside from clarity gains, this allows for massive speed improvements.

Also see this:

For strict mode functions, the values of the arguments object‘s properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

This is what NCZ means in context, quoting the book itself:

Strict mode makes several changes to how the arguments object can be used. First, assignment, as in the previous example, no longer works. The value of num2 remains undefined even though arguments[1] has been assigned to 10. Second, trying to overwrite the value of arguments is a syntax error. (The code will not execute.) - Page 82, Language Basics, Professional JavaScript, Nicholas C. Zakas

Named arguments and the arguments object in JavaScript

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