Proper way to set a function's default parameter to a boolean?

Question

I\'ve a function read() that takes as a boolean paramater. If false is passed in - read(false) - it shouldn\'t run a block of code. It wo

Answer 1

It's because the concept automatic conversion in Javascript, the undefined value convert to false. So three lines are similar to ensure the variable first_time_here is false, not undefined.

if first_time_here is undefined:

first_time_here = undedined !== false -> first_time_here = false != false
-> first_time_here = false;

And:

first_time_here = undedined || false -> first_time_here = false || false
-> first_time_here = false;

Answer 2

If you're expecting falsey values, use typeof:

var x = typeof x !== 'undefined' ? x : false;

Otherwise, if you do this:

var x = x || true;

And pass in false, the value of x will be true.