parameter-scope-examples.js

function f(x, x, …rest) {}
  Syntax Error: duplicate parameter name (Rule 1.A)

function f(x, {a:x, b:y}) {}
  Syntax Error: duplicate parameter name (Rule 1.A)

function f([x]) { let x; }
  Syntax Error: redeclaration of parameter x (Rule 1.B)

function f([x]) { var x; }
  Syntax Error: redeclaration of parameter x (Rule 1.B)

function f([x]) { {var x;} }
  Syntax Error: redeclaration of parameter x using hoisted var (Rule 1.B)

function f([x]) { {let x;} }
  Valid, redeclaration is in inner block

function f([x]) { function x(){} }
  Syntax Error: redeclaration of parameter x (Rule 1.B)

function f([x]) { class x {} }
  Syntax Error: redeclaration of parameter x (Rule 1.B)


function f(x, y=x) {}
  Valid, x has been initialized when y's default value expression is evaluated.

function f(x=y, y) {}
  Runtime: ReferenceError exception y not initialized (Rule 2)

const a = "A";
function f(x=a) {}
  Valid

function f(x=a) { var a;}
  Runtime: ReferenceError a not yet initialized (Rule 2.A)

function f(x=a) { const a = "A";}
  Runtime: ReferenceError a not yet initialized (Rule 2.A)

function a() {return "A";}
function f(x=a()) {}
  Valid, a is initialized in surrounding scope, at time of parameter default value initialization.

function f(x=a()) { function a() { return "A"; } }
  Runtime: ReferenceError a not yet initialized (Rule 2.A)

whiteboard.js

(6)
function f(x, y = 7) {
  ...
}

<===>

(5)
function f(x, y) {
  let x1 = x;
  let y1 = y ?? 7;
  
  (5)
  return (function(x, y) {
    ...
  }).call(this, x1, y1);
}


WH: (whiteboard)

function f(x, x=7, z=x) {
  let x1 = x;
  let x2 = x ?? 7;
  let z1 = z ?? which x?;
}