prefer-type-error.md

Enforce throwing TypeError in type checking conditions

This rule enforces you to throw a TypeError after a type checking if-statement, instead of a generic Error.

It's aware of the most commonly used type checking operators and identifiers like typeof, instanceof, .isString(), etc, borrowed from ES2017, Underscore, Lodash, and jQuery. For a complete list of the recognized identifiers, please take a look at the identifier-definition.

The rule investigates every throw-statement which throws a generic Error. It will fail if the throw-statement is the only expression in the surrounding block and is preceeded by an if-statement whose condition consists of type-checks exclusively. You have to replace the Error with a TypeError.

This rule is fixable.

Fail

if (Array.isArray(foo) === false) {
	throw new Error('Array expected');
}

if (Number.isNaN(foo) === false && Number.isInteger(foo) === false) {
	throw new Error('Integer expected');
}

if (isNaN(foo) === false) {
	throw new Error('Number expected');
}

if (typeof foo !== 'function' &&
	foo instanceof CookieMonster === false &&
	foo instanceof Unicorn === false) {
	throw new Error('Magic expected');
}

Pass

if (Array.isArray(foo) === false) {
	throw new TypeError('Array expected');
}

if (Number.isNaN(foo) === false && Number.isInteger(foo) === false) {
	throw new TypeError('Integer expected');
}

if (isNaN(foo) === false) {
	throw new TypeError('Number expected');
}

if (typeof foo !== 'function' &&
	foo instanceof CookieMonster === false &&
	foo instanceof Unicorn === false) {
	throw new TypeError('Magic expected');
}