For the month of December I have challenged myself to write 24 articles about some JS concepts up until Xmas.

The first installment in this series is around Truthy and Falsy values in Javascript.

What is a Truthy or Falsy value?

A truthy value is any value in Javascript thatr returns true when converted to a Boolean and a falsy value is any value in Javascript which is false when converted to a Boolean.

For example, a non-empty string is a truthy value as it returns true when comverted with the double NOT, whereas an empty string evaluates to false so it is a falsy value.

!!"test" === true;

!!"" === false;

And a slightly confusing example, even if the string contains a falsy value it will return true:

console.log(!!"0") // true
console.log(!!"false") // true

There are only a select few falsy values in Javascript. They are:

"", '', `` // empty strings
0 // zero (also minus 0)
null
NaN
false
undefined

• All other values in Javascript are truthy. Including empty arrays and objects.

The double NOT operator (!!)

• The double NOT operator will convert the value to a Boolean context and return a boolean value as in the examples above but can also be confusing.

For example then using on a Boolean value initially:

console.log(!!true);

This statement will negate true first to give us false and then negate false to give us true and then log that to the console.

So:

!!bVal === bVal

As an alternative you can also use the Boolean constructor:

const truthy = new Boolean("string");
console.log(truthy); // true

const falsy = new Boolean(0);
console.log(falsy); // false

AND and OR operators

• You can also use the AND and OR operators with truthy and falsy values, as they will be converted upon evaluation.

!!({} && "") // returns false

!!({} || "") // returns true

JS Advent #2 >