ecmascript 6 - Can we just use "if (a === undefined)" to handle default parameter values in JavaScript before ES6? -

in es6, can use

function f (x, y = 7, z = 42) {     return x + y + z }  console.log(f(1)); 

and both babel , traceur translate similar es5 code way:

"use strict";  function f(x) {     var y = arguments.length <= 1 || arguments[1] === undefined ? 7 : arguments[1];     var z = arguments.length <= 2 || arguments[2] === undefined ? 42 : arguments[2];      return x + y + z; }  console.log(f(1)); 

isn't true can handle it, on http://es6-features.org/#defaultparametervalues by:

function f (x, y, z) {     if (y === undefined)         y = 7;     if (z === undefined)         z = 42;     return x + y + z; } console.log(f(1)); 

even mozilla's website uses like:

var b = typeof b !== 'undefined' ?  b : 1; 

or a standard answer on so uses:

function foo(a, b)  {    = typeof !== 'undefined' ? : 42;    b = typeof b !== 'undefined' ? b : 'default_b';    ...  } 

why don't use short , simple

if (y === undefined)         y = 7; 

is there subtle difference?

(it true in global scope, can't a === undefined if a never declared (it raise error). inside of function, can use a === undefined because a listed on parameter list already. both babel , traceur don't list default parameter in parameter list, however.)

can use if (a === undefined) handle default parameter values in javascript before es6?

yes, can that, it's totally fine.

but right, there is subtle difference (or two):

• f.length expectedargumentcount, number of parameters before first rest or initialised one.
• in function f( a=b, b=0 ) variable b has temporal dead zone , must not used initialiser of a.

transpilers babel try replicate things these closely possible, that's why use arguments objects , declare variables in function body. btw, babel fails handle function f(a=0, b) correctly.