Wrap toString()

index.d.ts

@@ -14,6 +14,8 @@ Modifies the `to` function to mimic the `from` function. Returns the `to` functi
 
 `name`, `displayName`, and any other properties of `from` are copied. The `length` property is not copied. Properties present in `to` but not in `from` are deleted. Prototype, class, and inherited properties are copied.
 
+`to.toString()` will return the same as `from.toString()` but prepended with a `Wrapped with to()` comment.
+
 @param to - Mimicking function.
 @param from - Function to mimic.
 @returns The modified `to` function.

index.js

@@ -40,6 +40,22 @@ const changePrototype = (to, from) => {
 	Object.setPrototypeOf(to, fromPrototype);
 };
 
+const wrappedToString = (withName, fromBody) => `/* Wrapped ${withName}*/\n${fromBody}`;
+
+const toStringDescriptor = Object.getOwnPropertyDescriptor(Function.prototype, 'toString');
+const toStringName = Object.getOwnPropertyDescriptor(Function.prototype.toString, 'name');
+
+// We call `from.toString()` early (not lazily) to ensure `from` can be garbage collected.
+// We use `bind()` instead of a closure for the same reason.
+// Calling `from.toString()` early also allows caching it in case `to.toString()` is called several times.
+const changeToString = (to, from, name) => {
+	const withName = name === '' ? '' : `with ${name.trim()}() `;
+	const newToString = wrappedToString.bind(null, withName, from.toString());
+	// Ensure `to.toString.toString` is non-enumerable and has the same `same`
+	Object.defineProperty(newToString, 'name', toStringName);
+	Object.defineProperty(to, 'toString', {...toStringDescriptor, value: newToString});
+};
+
 // If `to` has properties that `from` does not have, remove them
 const removeProperty = (to, from, property, ignoreNonConfigurable) => {
 	if (hasOwnProperty.call(from, property)) {
@@ -56,6 +72,8 @@ const removeProperty = (to, from, property, ignoreNonConfigurable) => {
 };
 
 const mimicFn = (to, from, {ignoreNonConfigurable = false} = {}) => {
+	const {name} = to;
+
 	for (const property of Reflect.ownKeys(from)) {
 		copyProperty(to, from, property, ignoreNonConfigurable);
 	}
@@ -66,6 +84,8 @@ const mimicFn = (to, from, {ignoreNonConfigurable = false} = {}) => {
 		removeProperty(to, from, property, ignoreNonConfigurable);
 	}
 
+	changeToString(to, from, name);
+
 	return to;
 };
 

readme.md

@@ -34,6 +34,9 @@ console.log(wrapper.name);
 
 console.log(wrapper.unicorn);
 //=> '🦄'
+
+console.log(String(wrapper));
+//=> '/* Wrapped with wrapper() */\nfunction foo() {}'
 ```
 
 
@@ -45,6 +48,8 @@ Modifies the `to` function to mimic the `from` function. Returns the `to` functi
 
 `name`, `displayName`, and any other properties of `from` are copied. The `length` property is not copied. Properties present in `to` but not in `from` are deleted. Prototype, class, and inherited properties are copied.
 
+`to.toString()` will return the same as `from.toString()` but prepended with a `Wrapped with to()` comment.
+
 #### to
 
 Type: `Function`

test.js

@@ -55,10 +55,11 @@ test('should keep descriptors', t => {
 	const wrapper = function () {};
 	mimicFn(wrapper, foo);
 
-	const {length: fooLength, ...fooProperties} = Object.getOwnPropertyDescriptors(foo);
-	const {length: wrapperLength, ...wrapperProperties} = Object.getOwnPropertyDescriptors(wrapper);
+	const {length: fooLength, toString: fooToString, ...fooProperties} = Object.getOwnPropertyDescriptors(foo);
+	const {length: wrapperLength, toString: wrapperToString, ...wrapperProperties} = Object.getOwnPropertyDescriptors(wrapper);
 	t.deepEqual(fooProperties, wrapperProperties);
-	t.notDeepEqual(fooLength, wrapperLength);
+	t.not(fooLength, wrapperLength);
+	t.not(fooToString, wrapperToString);
 });
 
 test('should copy inherited properties', t => {
@@ -112,6 +113,76 @@ test('should allow classes to be copied', t => {
 	t.not(wrapperClass.prototype, fooClass.prototype);
 });
 
+test('should patch toString()', t => {
+	const wrapper = function () {};
+	mimicFn(wrapper, foo);
+
+	t.is(wrapper.toString(), `/* Wrapped with wrapper() */\n${foo.toString()}`);
+});
+
+test('should patch toString() with arrow functions', t => {
+	const wrapper = function () {};
+	const arrowFn = value => value;
+	mimicFn(wrapper, arrowFn);
+
+	t.is(wrapper.toString(), `/* Wrapped with wrapper() */\n${arrowFn.toString()}`);
+});
+
+test('should patch toString() with bound functions', t => {
+	const wrapper = function () {};
+	const boundFn = (() => {}).bind();
+	mimicFn(wrapper, boundFn);
+
+	t.is(wrapper.toString(), `/* Wrapped with wrapper() */\n${boundFn.toString()}`);
+});
+
+test('should patch toString() with new Function()', t => {
+	const wrapper = function () {};
+	// eslint-disable-next-line no-new-func
+	const newFn = new Function('');
+	mimicFn(wrapper, newFn);
+
+	t.is(wrapper.toString(), `/* Wrapped with wrapper() */\n${newFn.toString()}`);
+});
+
+test('should patch toString() several times', t => {
+	const wrapper = function () {};
+	const wrapperTwo = function () {};
+	mimicFn(wrapper, foo);
+	mimicFn(wrapperTwo, wrapper);
+
+	t.is(wrapperTwo.toString(), `/* Wrapped with wrapperTwo() */\n/* Wrapped with wrapper() */\n${foo.toString()}`);
+});
+
+test('should keep toString() non-enumerable', t => {
+	const wrapper = function () {};
+	mimicFn(wrapper, foo);
+
+	const {enumerable} = Object.getOwnPropertyDescriptor(wrapper, 'toString');
+	t.false(enumerable);
+});
+
+test('should print original function with Function.prototype.toString.call()', t => {
+	const wrapper = function () {};
+	mimicFn(wrapper, foo);
+
+	t.is(Function.prototype.toString.call(wrapper), 'function () {}');
+});
+
+test('should work with String()', t => {
+	const wrapper = function () {};
+	mimicFn(wrapper, foo);
+
+	t.is(String(wrapper), `/* Wrapped with wrapper() */\n${foo.toString()}`);
+});
+
+test('should not modify toString.name', t => {
+	const wrapper = function () {};
+	mimicFn(wrapper, foo);
+
+	t.is(wrapper.toString.name, 'toString');
+});
+
 // eslint-disable-next-line max-params
 const configurableTest = (t, shouldThrow, ignoreNonConfigurable, toDescriptor, fromDescriptor) => {
 	const wrapper = function () {};