Arguments and reference

Question

Consider this JavaScript function:

var f = function (a) {
  console.log(a+" "+arguments[0]);
  a = 3;
  console.log(a+" "+arguments[0]);
}

I would expect that a and arguments[0] reference the same value only up to the second statement of the function. Instead they appear to always refer the same value: f(2) causes

2 2
3 3

and f({foo: 'bar'}) causes:

[object Object] [object Object]
3 3

Are argument identifiers and the arguments identifier linked in a special way?

Answer 1

Are argument identifiers and the arguments identifier linked in a special way?

Yes (but only in non-strict mode).

From the specification (ES6, ES5):

For non-strict mode functions the integer indexed data properties of an arguments object whose numeric name values are less than the number of formal parameters of the corresponding function object initially share their values with the corresponding argument bindings in the function’s execution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into an accessor property. 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.