JavaScript: Converting Strings to Booleans

Converting strings to booleans in JavaScript comes in handy when you’re dealing with user inputs or API responses that tend to arrive as strings but represent boolean values. This post covers how to do this.

Understanding the basics of converting strings to booleans

In JavaScript, every value falls into one of two categories: "truthy" or "falsy". This categorization is the cornerstone of converting strings to booleans. JavaScript defines the following values as falsy:

• false
• 0
• "" (an empty string)
• null
• undefined
• NaN

Conversely, JavaScript treats all other values, including non-empty strings, as truthy. This distinction is important for string to boolean conversion.

How to use the Boolean function for explicit conversion?

To explicitly convert a string to a boolean, use the Boolean function. This method directly applies JavaScript's truthy and falsy rules, offering a clear path for conversion.

let trueString = "true";
let falseString = "";

let booleanTrue = Boolean(trueString); // true
let booleanFalse = Boolean(falseString); // false

Leveraging logical operators for implicit conversion

Another way to convert a string to a boolean is by using logical operators, like ! (logical NOT). Double application, !!, coerces a value to its boolean equivalent, reflecting JavaScript's interpretation of truthy and falsy values.

let trueString = "any non-empty string";
let falseString = "";

let booleanTrue = !!trueString; // true
let booleanFalse = !!falseString; // false

Converting specific strings to booleans

If you want to convert specific strings like "true" or "false" to their boolean equivalents, you’ll need to craft a custom function. That’s because JavaScript doesn't automatically map these strings to boolean values.

function stringToBoolean(string) {
    switch(string.toLowerCase().trim()) {
        case "true":
            return true;
        case "false":
            return false;
        default:
            throw new Error("String does not represent a boolean value");
    }
}

let trueBoolean = stringToBoolean("true"); // true
let falseBoolean = stringToBoolean("false"); // false