Boolean Data Type

Boolean represents a logical entity. Logical entities are used to write conditions and enable branching in the program. There are only 2 values in JavaScript that are of Boolean type. They are true and false.

// Branching
if (true) {
  console.log("I am truthy");
} else {
  console.log("I will never be logged");

Falsy values

When we use any data types as a logical entity, JavaScript automatically converts that value to a boolean true or false. For example, in the following snippet we use "hello" as a logical entity.

if ("hello") {
  console.log("hello is true");
} else {
  console.log("hello is false");

Here the output is "hello is true". It is because, JavaScript automatically converts a non-empty string to a true. There are some values, when converted to boolean, results in false. Those values are called falsy values. They are:

  • 0
  • -0
  • null
  • false Obviously
  • NaN
  • undefined
  • "" Empty string

All other values in JavaScript are truthy values.

An empty object {} and empty array [] are truthy values. It is not falsy like an empty string.

The string "false" is a non-empty string. So when it is converted to boolean, it is a truthy value.

Converting any value to boolean

We can convert any value in JavaScript to boolean type. This can be done using 2 techniques.

Boolean function

We can use Boolean function to convert any values to boolean type.

console.log(Boolean("A string")); // true
console.log(Boolean(""));         // false
console.log(Boolean(0));          // false
console.log(Boolean({}));         // true

!! Double negation

In JavaScript ! can be used as a NOT operator. In Mathematics, if we have a number 23, negating it results in -23. Again negating it reverts it back to 23. In similar way, in JavaScript when using !, it negates the value and the output is a boolean value.

Here is an example. We know that, a non-empty string is a truthy value. Let us do a single negation first.

console.log(!"hello"); // false

We use this behaviour of ! to convert a value to boolean type by negating again.

console.log(!!"hello"); // true