Convert a String to a Boolean in TypeScript

Convert a String to a Boolean in TypeScript #

To convert a string to a boolean in TypeScript, use the strict equality operator to compare the string to the string "true", e.g. const bool = str === 'true'. If the condition is met, the strict equality operator will return the boolean value true, otherwise false is returned.

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const str1 = 'true';

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

// ✅ If the string might be Uppercase  or Title Case
const str2 = 'False';

// 👇️ const bool2: boolean
const bool2 = str2.toLowerCase() === 'true';
console.log(bool2); // 👉️ false

The strict equality (===) operator checks if the values to the left and right hand side are equal and returns true if they are, and false otherwise.

To convert any other string to a boolean, use the Boolean object.

Convert a String to a Boolean using Boolean in TypeScript #

If you need to convert a string to a boolean and consider only empty string values to be false and all other string values to be true, use the Boolean object.

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const bool3 = Boolean('hello'); // 👉️ true
const bool4 = Boolean(''); // 👉️ false
const bool5 = Boolean('false'); // 👉️ true
const bool6 = Boolean('true'); // 👉️ true
const bool7 = Boolean(' '); // 👉️ true

The Boolean object converts the passed in string to a boolean. Empty strings get converted to false, in all other cases the result is true.

All other strings convert to true.

This is because the Boolean objects converts truthy values to true and falsy values to false.

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

Notice that the empty string is a falsy value, this is why it gets converted to false.

If the string contains at least 1 character, be it an empty space, it's truthy and gets converted to true.

A more concise approach, which achieves the same result is to use the double NOT (!!) operator.

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const bool8 = !!'hello'; // 👉️ true
const bool9 = !!''; // 👉️ false
const bool10 = !!'false'; // 👉️ true
const bool11 = !!'true'; // 👉️ true
const bool12 = !!' '; // 👉️ true

The double NOT (!!) operator converts empty strings to false, all other strings get converted to the boolean value true.

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

The logical NOT (!) operator converts a value to a boolean and inverts the result. Here are some examples.

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

The empty string is a falsy value, but when converted to boolean and flipped, we get true back.

To perform a pure boolean conversion, we have to flip the value again by using a second logical NOT (!) operator.

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

The double NOT (!!) operator is quite concise and does the same thing the Boolean object does, however it's a bit harder to read if you're not familiar with the logical NOT (!) operator.

Both ways of converting a string to a boolean are very common and often used throughout the same codebase.