Default parameter is used ONLY when it's `undefined`. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters

test_default_parameters.js

function test(
	a = '_a default_\n',
	b = '_b default_\n',
	c = '_c default_\n',
	d = '_d default_\n',
	e = '_e default_\n',
	f = '_f default_\n'
) {
	console.log(a, b, c, d, e, f);
}

test(
	undefined,	// a
	null,		// b
	false,		// c
	[],		// d
	{},		// e
	''		// f
);

/* 
  output:
» node test6.js
_a default_
 null false [] {} 

 */