How to convert a Value to a Boolean in TypeScript

Convert a Value to a Boolean in TypeScript #

Use the Boolean object to convert a value to a boolean in Typescript, e.g. Boolean(someValue). When used as a function, the Boolean object converts the passed in value to a boolean - true if the value is truthy and false if it's a falsy value.

Copied!
const str1 = 'hello';

// 👇️ const bool1: boolean
const bool1 = Boolean(str1);
console.log(bool1); // 👉️ true

We used the Boolean object to convert a value to a boolean.

The boolean object returns false if the passed in value is falsy.

The falsy values in JavaScript are: false, 0, "" (empty string), null, undefined, NaN (not a number).

In all other cases, true is returned.

Here are some examples of using the Boolean object.

Copied!
console.log(Boolean([])); // 👉️ true
console.log(Boolean({})); // 👉️ true
console.log(Boolean('')); // 👉️ false
console.log(Boolean('false')); // 👉️ true
console.log(Boolean(0)); // 👉️ false
console.log(Boolean(10)); // 👉️ true

Note that an empty array and empty object are truthy values, whereas an empty string is falsy.

Copied!
// 👇️ const bool1: true
const bool1 = !!'hello';
console.log(bool1); // 👉️ true

// 👇️ const bool1: false
const bool2 = !!'';
console.log(bool2); // 👉️ false

Using the double bang (!!) operator is the same as using the logical NOT (!) operator twice.

The logical NOT (!) operator converts a value into a boolean and inverts the result.

Copied!
console.log(!'hello'); // 👉️ false
console.log(!''); // 👉️ true

When we use the logical NOT (!) operator twice, we:

• convert the value to a boolean and flip it
• flip the boolean again

An easy way to think about it is - we're basically converting a value to a boolean.

Here are some examples.

Copied!
console.log(!![]); // 👉️ true
console.log(!!{}); // 👉️ true
console.log(!!''); // 👉️ false
console.log(!!'false'); // 👉️ true
console.log(!!0); // 👉️ false
console.log(!!10); // 👉️ true