How to Check if the Type of a JavaScript Variable is Boolean?

Since JavaScript is a dynamically typed language, its variables can contain data of any type.

Therefore, we need a way to check for the data type of a variable.

In this article, we’ll look at ways to check if the data type of a JavaScript variable is a boolean.

The typeof Operator

One way to check if a variable is a boolean variable is using the typeof operator.

To do this, we write:

if (typeof variable === "boolean") {
  // ...
}

We check if the data of the variable variable is a boolean with the expression if the if statement.

It’ll return true if variable is a boolean.

The === Operator

Another way to check is a variable is a boolean is to check if it equals to true or false with the === operator.

For instance, we can write:

const isBoolean = (val) => {
  return val === false || val === true;
}
console.log(isBoolean(true))
console.log(isBoolean('abc'))

We create the isBoolean that checks whether val is false or true .

Since we use the === operator, either expression in the OR expression will return true only if val is exactly equal to the value being compared.

Therefore, the first console log should log true .

And the 2nd one should log false .

Check the String Representation of the Variable with toString

In addition to checking if the value is exactly equal to true or false , we can also compare the string value of it to see if it’s a boolean.

For instance, we can write:

const isBoolean = (val) => {
  return val === true || val === false || toString.call(val) === '[object Boolean]';
}
console.log(isBoolean(true))
console.log(isBoolean('abc'))

We call toString.call with our val parameter to see if it returns ‘[object Boolean]’ .

If it does, then it gives us more confidence that val is a boolean.

Using the valueOf Method

In case that we have boolean variables that are boxed with the Boolean constructor, we can also call the valueOf method to return the primitive version of the boolean variable.

For instance, we can write:

const isBoolean = (val) => {
  return typeof val === 'boolean' ||
    (
      typeof val === 'object' &&
      val !== null &&
      typeof val.valueOf() === 'boolean'
    );
}
console.log(isBoolean(true))
console.log(isBoolean('abc'))

to add:

(
  typeof val === 'object' &&
  val !== null &&
  typeof val.valueOf() === 'boolean'
)

to check if val is an object that isn’t null but the when we call valueOf it, we get 'boolean' returned.

This expression would be true if we invoke the Boolean constructor by writing expressions like:

new Boolean(true)

to create a boxed boolean object.

The console logs should be the same as the previous examples.

Conclusion

There’re many ways we can use to check if a JavaScript variable is a boolean variable.