JS default argument value from variable: why must identifier be different? [duplicate]

Question

This question already has an answer here:

• Scope of Default function parameters in javascript 1 answer

Assigning a default value using a variable with the same name throw a reference error:

var a = 'adef';
var x = (a=a) => console.log(a);
x();
=> "ReferenceError: a is not defined"

But this is fine:

var other = 'otherdef';
var x = (a=other) => console.log(a);
x();
=> "otherdef"

My assumption was that the value of a in the outer scope would be assigned to the new scope.

I have tried using const instead of var, and class/function instead of an arrow-function, but the result is always the same (tested in chrome 63 and node 6).

I have a feeling the issue is that a is 'hoisted' during assignment and so the assignment is referring to the new 'a' (which exists but is undefined)...

Answer 1

The purpose of this behavior is to allow a parameter to be default-initialized to the value of another parameter, e.g:

var a = 2;
var x = (a, b = a) => console.log(a, b);
x(42); // 42 42

Making the special case a = a work differently could be done but that would make it harder to refactor functions that use this behavior (you wouldn't be able to rename the parameter a without also renaming the variable that it depends on).