How can I convert a string to a boolean in JavaScript?

Matthew C.

—January 30, 2023

The Problem

You have a string that you want to convert to a boolean. You may have a boolean value that gets converted to a string, for example when values are submitted via a form. How do you convert a “true” or “false” string to a boolean?

The Solution

You can use an equality operator, Boolean() constructor, or the double NOT (!!) operator to convert a “true” or “false” string to a boolean.

Using the Strict Equality Operator (===)

You can parse the string to a boolean using the strict equality operator(===). You can check if the value is equal to “true”:

let trueStr = "true";
trueStr = (trueStr === "true");
console.log(trueStr, typeof trueStr); // true "boolean"

This would also work using the equality operator (==), which will try to coerce the values being compared to the same type before comparing them. It’s best to use the strict equality operator (===), as the type conversion of the equality operator (==) can lead to unexpected results:

console.log("" === "0"); // false
console.log(0 == ""); // true
console.log(0 == "0"); // true
console.log(false == "false"); // false
console.log(false == "0"); // true
console.log(false == ""); // true

You can also use the toLowerCase() method to do a case-insensitive check for “true”:

let trueStr = "true";
trueStr = (trueStr.toLowerCase() === "true");
console.log(trueStr, typeof trueStr); // true "boolean"

The problem with the above code is that it will throw an error if the trueStr variable is null or undefined:

Cannot read properties of undefined (reading 'toLowerCase')

There are two fixes for this. You can use the typeOf operator to check that the value is a string before lower casing the value:

let trueStr = null;
trueStr = (typeof trueStr === "string" && trueStr.toLowerCase() === "true");
console.log(trueStr, typeof trueStr); // false "boolean"

You can also use an if statement to first check if the trueStr variable is defined:

let trueStr = null;
if (trueStr) {
  trueStr = (trueStr.toLowerCase() === "true");
}
console.log(trueStr, typeof trueStr); // null "object"

Using the Boolean() Constructor or the Double NOT (!!) Operator

You can use the Boolean() constructor to create a boolean object and then use its valueOf property to get the primitive boolean value. It takes in a value to convert to a boolean. If the value is falsy it will return false. If the value is truthy, it will return true. All values are truthy except: false, 0, -0, 0n, "", null, undefined, and NaN.

The double NOT (!!) operator coerces a value to a primitive boolean value, like the Boolean() constructor:

const str1 = "true";
const str2 = "false";
const str3 = "";
const str4 = " ";

console.log(!!str1); // true
console.log(!!str2); // true
console.log(!!str3); // false
console.log(!!str4); // true

console.log(Boolean(str1).valueOf()); // true
console.log(Boolean(str2).valueOf()); // true
console.log(Boolean(str3).valueOf()); // false
console.log(Boolean(str4).valueOf()); // true

These methods are not ideal as “false” returns true because the only falsy string is an empty string.