Add method for getting call count of a function (#5)

Fixes #4

index.js

@@ -1,6 +1,8 @@
 'use strict';
 const mimicFn = require('mimic-fn');
 
+const calledFns = new WeakMap();
+
 module.exports = (fn, opts) => {
 	// TODO: Remove this in v3
 	if (opts === true) {
@@ -15,9 +17,12 @@ module.exports = (fn, opts) => {
 
 	let ret;
 	let called = false;
+	let count = 0;
 	const fnName = fn.displayName || fn.name || '<anonymous>';
 
 	const onetime = function () {
+		calledFns.set(onetime, count++);
+
 		if (called) {
 			if (opts.throw === true) {
 				throw new Error(`Function \`${fnName}\` can only be called once`);
@@ -34,6 +39,9 @@ module.exports = (fn, opts) => {
 	};
 
 	mimicFn(onetime, fn);
+	calledFns.set(onetime, count++);
 
 	return onetime;
 };
+
+module.exports.callCount = fn => calledFns.get(fn);

readme.md

@@ -24,6 +24,8 @@ const foo = onetime(() => i++);
 foo(); //=> 0
 foo(); //=> 0
 foo(); //=> 0
+
+onetime.callCount(foo); //=> 3
 ```
 
 ```js
@@ -59,6 +61,16 @@ Default: `false`
 
 Throw an error when called more than once.
 
+## onetime.callCount(fn)
+
+Returns a number representing how many times `fn` is called.
+
+#### fn
+
+Type: `Function`
+
+Function to get call count from.
+
 
 ## License
 

test.js

@@ -14,3 +14,11 @@ test('option to throw is called more than once', t => {
 	f();
 	t.throws(f, /Function .* can only be called once/);
 });
+
+test('`callCount` method', t => {
+	const f = m(() => {});
+	f();
+	f();
+	f();
+	t.is(m.callCount(f), 3);
+});