pseudo code to show what I'm looking for

lib/index.js

@@ -14,7 +14,6 @@ import rimraf from 'rimraf';
 import screenshotStream from 'screenshot-stream';
 import viewportList from 'viewport-list';
 import protocolify from 'protocolify';
-import arrayUniq from 'array-uniq';
 import filenamifyUrl from 'filenamify-url';
 import template from 'lodash.template';
 import pify from 'pify';
@@ -55,11 +54,20 @@ type Viewport = {
 	keywords: Array<string>;
 };
 
+type srcFn<Value> =
+	& ((_: void) => void) // TODO: should return `this._src`
+	& ((url: string, sizes: Array<string>, options?: Options) => Value); // TODO: should return `this`
+
+type destFn<Value> =
+	& ((_: void) => void) // TODO: should return `this._dest`
+	& ((dir: string) => Value); // TODO: should return `this`
+
 const getResMem = mem(getRes);
 const viewportListMem = mem(viewportList);
 
 let listener;
-export default class Pageres extends EventEmitter {
+
+export default class Pageres<Value> extends EventEmitter {
 	/**
 	 * Initialize a new Pageres
 	 *
@@ -96,9 +104,8 @@ export default class Pageres extends EventEmitter {
 	 * @param {Object} options
 	 * @api public
 	 */
-
-	// src(url?: string, sizes?: Array<string>, options?: Options): Pageres | Array<Src> {
-	src(url, sizes, options): Pageres | Array<Src> { // TODO: how can I make all the arguments optional or all required? the commented out line doesn't work as it complains it can also be `undefined`. Can I define a separate type definiton for each scenario?
+	src: srcFn<Value>;
+	src(url, sizes, options) { // TODO: how can I make all the arguments optional or all required? the commented out line doesn't work as it complains it can also be `undefined`. Can I define a separate type definiton for each scenario?
 		if (!arguments.length) {
 			return this._src;
 		}
@@ -114,8 +121,8 @@ export default class Pageres extends EventEmitter {
 	 * @param {String} dir
 	 * @api public
 	 */
-
-	dest(dir): Pageres | string { // TODO: is there any way to define retur types based on input?
+	dest: destFn<Value>;
+	dest(dir) {
 		if (!arguments.length) {
 			return this._dest;
 		}