Convert a string to a boolean

Sometimes, one might run into the problem of converting the string representation of a value into the value itself. This is often straightforward, such as with numeric values. However, string representations of boolean values can be a bit trickier.

This issue arises due to the fact that any non-empty string is considered truthy in JavaScript. On top of that, strings can have different capitalization or whitespace, making it harder to compare them directly to a constant.

Truthy and falsy values

JavaScript uses type coercion in Boolean contexts, resulting in truthy or falsy values. Get a hang of how it all works in this quick guide.

To counteract this, it's often a good idea to use a couple of transformations, namely String.prototype.toLowerCase() and String.prototype.trim(), to make the string representation of the value more consistent. Additionally, an array of acceptable values might make it easier to perform the conversion in certain cases.

const toBoolean = (value, truthyValues = ['true']) => {
  const normalizedValue = String(value).toLowerCase().trim();
  return truthyValues.includes(normalizedValue);
};

toBoolean('true'); // true
toBoolean('TRUE'); // true
toBoolean('True'); // true
toBoolean('tRue '); // true
toBoolean('false'); // false
toBoolean('FALSE'); // false
toBoolean('False'); // false
toBoolean('fAlse '); // false
toBoolean('YES', ['yes']); // true
toBoolean('no', ['yes']); // false