Improve type system for get and head methods

index.d.ts

@@ -1,3 +1,5 @@
+type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
+
 type JSONObject = {[key: string]: JSONValue};
 interface JSONArray extends Array<JSONValue> {}
 export type JSONValue = string | number | boolean | null | JSONObject | JSONArray;
@@ -76,6 +78,14 @@ export interface Options extends RequestInit {
 	throwHttpErrors?: boolean;
 }
 
+interface OptionsWithoutBody extends Omit<Options, 'body'> {
+	method?: 'get' | 'head'
+}
+
+interface OptionsWithBody extends Options {
+	method?: 'post' | 'put' | 'delete'
+}
+
 /**
  * Returns a `Response` object with `Body` methods added for convenience.
  */
@@ -106,15 +116,15 @@ export interface Ky {
 	 * @param input - `Request` object, `URL` object, or URL string.
 	 * @returns Promise with `Body` method added.
 	 */
-	(input: Request | URL | string, options?: Options): ResponsePromise;
+	(input: Request | URL | string, options?: OptionsWithoutBody | OptionsWithBody): ResponsePromise;
 
 	/**
 	 * Fetches the `input` URL with the option `{method: 'get'}`.
 	 *
 	 * @param input - `Request` object, `URL` object, or URL string.
 	 * @returns Promise with `Body` method added.
 	 */
-	get(input: Request | URL | string, options?: Options): ResponsePromise;
+	get(input: Request | URL | string, options?: Omit<Options, 'body'>): ResponsePromise;
 
 	/**
 	 * Fetches the `input` URL with the option `{method: 'post'}`.
@@ -146,7 +156,7 @@ export interface Ky {
 	 * @param input - `Request` object, `URL` object, or URL string.
 	 * @returns Promise with `Body` method added.
 	 */
-	head(input: Request | URL | string, options?: Options): ResponsePromise;
+	head(input: Request | URL | string, options?: Omit<Options, 'body'>): ResponsePromise;
 
 	/**
 	 * Fetches the `input` URL with the option `{method: 'delete'}`.

index.js

@@ -168,6 +168,10 @@ class Ky {
 		const headers = new Headers(this._options.headers || {});
 
 		if (json) {
+			if (this._options.body) {
+				throw new TypeError('The `json` option cannot be used with the `body` option');
+			}
+
 			headers.set('content-type', 'application/json');
 			this._options.body = JSON.stringify(json);
 		}

test/main.js

@@ -108,6 +108,10 @@ test('POST JSON', async t => {
 	await server.close();
 });
 
+test('cannot use `json` option along with the `body` option', t => {
+	t.throws(() => ky('https://example.com', {json: {foo: 'bar'}, body: 'foobar'}), 'The `json` option cannot be used with the `body` option');
+});
+
 test('custom headers', async t => {
 	const server = await createTestServer();
 	server.get('/', (request, response) => {