Misc

Five predicates for specific built-in object types.

Predicate	Quick rule
isPromise(v)	v instanceof Promise (recognizes subclasses)
isDate(v)	typeof v === "object" && v instanceof Date
isGenerator(v)	Object with .next() AND [Symbol.iterator]() === v (duck-typed)
isFormElement(v)	v instanceof HTMLFormElement
isFormData(v)	v instanceof FormData

isPromise

isPromise(Promise.resolve());              // true
isPromise(new Promise((r) => r(1)));       // true
isPromise(fetch("/api"));                  // true

isPromise({ then() {} });                  // false  (thenable, not a Promise)
isPromise(async function () {}());         // true   (async fn returns a Promise)
isPromise({});                             // false
isPromise(null);                           // false

class MyPromise extends Promise<unknown> {}
isPromise(new MyPromise((r) => r(1)));     // true   (subclasses match via instanceof)

isDate

isDate(new Date());            // true
isDate(new Date(0));           // true
isDate(new Date("2024-01-01"));// true
isDate(new Date("not real"));  // true   (invalid Date is still a Date)

isDate("2024-01-01");          // false  (string, not Date)
isDate(Date.now());            // false  (number)
isDate({});                    // false
isDate(null);                  // false

Note: isDate(new Date("invalid")) is true. To check “is it a valid Date instance”:

function isValidDate(v: unknown): v is Date {
  return isDate(v) && !Number.isNaN((v as Date).getTime());
}

isGenerator

Duck-typed: an object that has a .next function AND whose [Symbol.iterator]() returns itself (the defining trait of a generator instance).

function* gen() { yield 1; }
isGenerator(gen());            // true   (generator instance)

isGenerator(gen);              // false  (the generator FUNCTION, not an instance)
isGenerator({});               // false
isGenerator(() => {});         // false
isGenerator("hello");          // false
isGenerator(null);             // false

isFormElement (Is.form)

Checks value instanceof HTMLFormElement. Guards on typeof HTMLFormElement === "undefined" so it returns false on the server instead of throwing.

const form = document.createElement("form");
isFormElement(form);              // true
isFormElement(document.createElement("div")); // false
isFormElement({});                // false
isFormElement(null);              // false

Aliased as Is.form and Is.formElement — both point at the same function.

isFormData

Checks value instanceof FormData. Throws on the server if FormData is undefined (it isn’t on Node 18+, where FormData is global).

isFormData(new FormData());       // true
isFormData({});                   // false
isFormData(null);                 // false

Notes

• isPromise, isDate, isFormElement, and isFormData all use instanceof, so subclasses match. isGenerator is duck-typed and matches generator instances regardless of how they were produced.
• All five return a real false (not the operand) for non-matches — safe to compare with === false or !.
• For Promise-shaped checks (the thenable protocol — anything with a .then(onFulfill, onReject) method), use value != null && typeof value.then === "function" directly. isPromise is stricter on purpose.