JavaScript Boolean: Truthy Values Are Not Parsed Flags

The task

Convert a few values, then parse a configuration field that accepts only actual Booleans or the exact strings true and false. Reject other input instead of guessing.

Runnable Node 22 example

const assert = require('node:assert/strict');

assert.equal(Boolean('false'), true);
assert.equal(Boolean(''), false);
assert.equal(Boolean([]), true);
assert.equal(Boolean(0), false);
function parseFlag(value) {
  if (value === true || value === 'true') return true;
  if (value === false || value === 'false') return false;
  throw new TypeError('expected Boolean or exact flag text');
}
assert.equal(parseFlag('false'), false);
assert.equal(parseFlag(true), true);
assert.throws(() => parseFlag('FALSE'), TypeError);
console.log(`coerced=${Boolean('false')}; parsed=${parseFlag('false')}`);

Expected output and explanation

The two printed results answer different questions. Coercion sees a nonempty string; the parser recognizes an explicitly supported spelling. The array assertion also shows that an empty array is truthy, so truthiness is not a container-size check.

The parser deliberately rejects uppercase and whitespace variants. If your input format permits them, normalize only as specified and test those additional cases. Returning a failure or throwing is preferable to quietly turning an unknown spelling into a false flag.

Pitfall to check

Boolean wrappers created with new Boolean are objects and can be truthy even when they wrap false. Prefer primitive Boolean values for flags. Parsing one value is also not a complete feature-flag system with precedence and authorization rules.