How to set default boolean values in JavaScript?

Question

Setting default optional values in JavaScript is usually done via the || character

var Car = function(color) {   this.color = color || 'blue'; };  var myCar = new Car(); console.log(myCar.color); // 'blue'  var myOtherCar = new Car('yellow'); console.log(myOtherCar.color); // 'yellow' 

That works because color is undefined and undefined || String is always the String. Of course that also works the other way around String || undefined is String. When two Strings are present the first one wins 'this' || 'that' is 'this'. It does NOT work the other way around as 'that' || 'this' is 'that'.

The question is: How can I achieve the same with boolean values?

Take the following example

var Car = function(hasWheels) {   this.hasWheels = hasWheels || true; }  var myCar = new Car(); console.log(myCar.hasWheels); // true  var myOtherCar = new Car(false) console.log(myOtherCar.hasWheels); // ALSO true !!!!!! 

For myCar it works because undefined || true is true but as you can see it does NOT work for myOtherCar because false || true is true. Changing the order doesn't help as true || false is still true.

Therefore, am I missing something here or is the following the only way to set the default value?

this.hasWheels = (hasWheels === false) ? false: true 

Cheers!

Answer 1

You can do this:

this.hasWheels = hasWheels !== false; 

That gets you a true value except when hasWheels is explicitly false. (Other falsy values, including null and undefined, will result in true, which I think is what you want.)

Answer 2

How about:

this.hasWheels = (typeof hasWheels !== 'undefined') ? hasWheels : true; 

Your other option is:

this.hasWheels = arguments.length > 0 ? hasWheels : true;