Truthy

JavaScript has a concept of truthy i.e. things that evaluate like true would in certain positions (e.g. if conditions and the boolean && || operators). The following things are truthy in JavaScript. An example is any number other than 0 e.g.

  1. if (123) { // Will be treated like `true`
  2.   console.log('Any number other than 0 is truthy');
  3. }

Something that isn’t truthy is called falsy.

Here’s a handy table for your reference.

Variable Type When it is falsy When it is truthy
boolean false true
string '' (empty string) any other string
number 0 NaN any other number
null always never
undefined always never
Any other Object including empty ones like {},[] never always

Being explicit

The !! pattern

Quite commonly it helps to be explicit that the intent is to treat the value as a boolean and convert it into a true boolean (one of true|false). You can easily convert values to a true boolean by prefixing it with !! e.g. !!foo. Its just ! used twice. The first ! converts the variable (in this case foo) to a boolean but inverts the logic (truthy -!> false, falsy -!> true). The second one toggles it again to match the nature of the original object (e.g. truthy -!> false -!> true).

It is common to use this pattern in lots of places e.g.

  1. // Direct variables
  2. const hasName = !!name;
  4. // As members of objects
  5. const someObj = {
  6.   hasName: !!name
  7. }
  9. // e.g. ReactJS
  10. {!!someName && <div>{someName}</div>}