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.
if (123) { // Will be treated like `true`
console.log('Any number other than 0 is truthy');
}
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.
// Direct variables
const hasName = !!name;
// As members of objects
const someObj = {
hasName: !!name
}
// e.g. ReactJS
{!!someName && <div>{someName}</div>}