Changing JavaScript function's parameter value using arguments array not working

Assigning to arguments indicies will only change the associated argument value (let's call it the n-th argument) if the function was called with at least n arguments. The arguments object's numeric-indexed properties are essentially setters (and getters):

http://es5.github.io/#x10.6

Italics in the below are my comments on how the process relates to the question:

(Let) args (be) the actual arguments passed to the [[Call]] internal method

1. Let len be the number of elements in args.

2. Let indx = len - 1.

3. Repeat while indx >= 0, (so, the below loop will not run when no arguments are passed to the function:)

(assign to the arguments object being created, here called map:)

• 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.

So, if the function is invoked with no arguments, there will not be a setter on arguments[0], so reassigning it won't change the parameter at index 0.

The same sort of thing occurs for other indicies as well - if you invoke a function with 1 parameter, but the function accepts two parameters, assigning to arguments[1] will not change the second parameter, because arguments[1] does not have a setter:

function fn(a, b) {  arguments[1] = 'bar';  console.log(b);}fn('foo');