add is.formData

source/index.ts

@@ -47,6 +47,7 @@ const objectTypeNames = [
 	'DataView',
 	'Promise',
 	'URL',
+	'FormData',
 	'HTMLElement',
 	...typedArrayTypeNames
 ] as const;
@@ -362,6 +363,7 @@ is.nonEmptyMap = <Key = unknown, Value = unknown>(value: unknown): value is Map<
 
 // `PropertyKey` is any value that can be used as an object key (string, number, or symbol)
 is.propertyKey = (value: unknown): value is PropertyKey => is.any([is.string, is.number, is.symbol], value);
+is.formData = (value: unknown): value is FormData => isObjectOfType<FormData>('FormData')(value);
 
 export type Predicate = (value: unknown) => boolean;
 
@@ -523,6 +525,7 @@ interface Assert {
 	emptyMap: (value: unknown) => asserts value is Map<never, never>;
 	nonEmptyMap: <Key = unknown, Value = unknown>(value: unknown) => asserts value is Map<Key, Value>;
 	propertyKey: (value: unknown) => asserts value is PropertyKey;
+	formData: (value: unknown) => asserts value is FormData;
 
 	// Numbers.
 	evenInteger: (value: number) => asserts value is number;
@@ -621,6 +624,7 @@ export const assert: Assert = {
 	emptyMap: (value: unknown): asserts value is Map<never, never> => assertType(is.emptyMap(value), AssertionTypeDescription.emptyMap, value),
 	nonEmptyMap: <Key = unknown, Value = unknown>(value: unknown): asserts value is Map<Key, Value> => assertType(is.nonEmptyMap(value), AssertionTypeDescription.nonEmptyMap, value),
 	propertyKey: (value: unknown): asserts value is number => assertType(is.propertyKey(value), 'PropertyKey', value),
+	formData: (value: unknown): asserts value is FormData => assertType(is.formData(value), 'FormData', value),
 
 	// Numbers.
 	evenInteger: (value: number): asserts value is number => assertType(is.evenInteger(value), AssertionTypeDescription.evenInteger, value),

test/test.ts

@@ -1621,6 +1621,27 @@ test('is.all', t => {
 	});
 });
 
+test('is.formData', t => {
+	const data = new window.FormData();
+	t.true(is.formData(data));
+	t.false(is.formData({}));
+	t.false(is.formData(undefined));
+	t.false(is.formData(null));
+
+	t.notThrows(() => {
+		assert.formData(data);
+	});
+	t.throws(() => {
+		assert.formData({});
+	});
+	t.throws(() => {
+		assert.formData(undefined);
+	});
+	t.throws(() => {
+		assert.formData(null);
+	});
+});
+
 test('assert', t => {
 	// Contrived test showing that TypeScript acknowledges the type assertion in `assert.number()`.
 	// Real--world usage includes asserting user input, but here we use a random number/string generator.