Add mapObject.mapObjectSkip for removing object keys (#38)

index.d.ts

@@ -11,7 +11,7 @@ declare namespace mapObject {
 		targetKey: MappedObjectKeyType,
 		targetValue: MappedObjectValueType,
 		mapperOptions?: mapObject.MapperOptions
-	];
+	] | typeof mapObject.mapObjectSkip;
 
 	interface Options {
 		/**
@@ -45,8 +45,28 @@ declare namespace mapObject {
 		*/
 		shouldRecurse?: boolean;
 	}
+
+	/**
+	Return this value from a `mapper` function to remove a key from an object.
+
+	```js
+	const mapObject = require('map-obj');
+
+	const object = {one: 1, two: 2}
+	const mapper = (key, value) => value === 1 ? [key, value] : mapObject.mapObjectSkip
+	const result = mapObject(object, mapper);
+
+	console.log(result);
+	//=> {one: 1}
+	```
+	*/
+	const mapObjectSkip: typeof uniqueSymbol
 }
 
+// unique symbol cannot be declared in a namespace directly, so we declare it top-level
+// See: https://github.com/sindresorhus/map-obj/pull/38#discussion_r702396878
+declare const uniqueSymbol: unique symbol;
+
 /**
 Map object keys and values into a new object.
 
@@ -65,6 +85,9 @@ const newObject = mapObject({FOO: true, bAr: {bAz: true}}, (key, value) => [key.
 
 const newObject = mapObject({FOO: true, bAr: {bAz: true}}, (key, value) => [key.toLowerCase(), value], {deep: true});
 //=> {foo: true, bar: {baz: true}}
+
+const newObject = mapObject({one: 1, two: 2}, (key, value) => value === 1 ? [key, value] : mapObject.mapObjectSkip);
+//=> {one: 1}
 ```
 */
 declare function mapObject<

index.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const isObject = value => typeof value === 'object' && value !== null;
+const mapObjectSkip = Symbol('skip');
 
 // Customized for this use-case
 const isObjectCustom = value =>
@@ -31,7 +32,13 @@ const mapObject = (object, mapper, options, isSeen = new WeakMap()) => {
 	}
 
 	for (const [key, value] of Object.entries(object)) {
-		let [newKey, newValue, {shouldRecurse = true} = {}] = mapper(key, value, object);
+		const mapResult = mapper(key, value, object);
+
+		if (mapResult === mapObjectSkip) {
+			continue;
+		}
+
+		let [newKey, newValue, {shouldRecurse = true} = {}] = mapResult;
 
 		// Drop `__proto__` keys.
 		if (newKey === '__proto__') {
@@ -57,3 +64,5 @@ module.exports = (object, mapper, options) => {
 
 	return mapObject(object, mapper, options);
 };
+
+module.exports.mapObjectSkip = mapObjectSkip;

index.test-d.ts

@@ -34,3 +34,5 @@ expectAssignable<{[key: string]: unknown}>(object3);
 expectType<'baz'>(object3.bar);
 
 mapObject({foo: 'bar'}, (key, value) => [value, key, {shouldRecurse: false}]);
+
+mapObject({foo: 'bar'}, () => mapObject.mapObjectSkip);

readme.md

@@ -21,6 +21,9 @@ const newObject = mapObject({FOO: true, bAr: {bAz: true}}, (key, value) => [key.
 
 const newObject = mapObject({FOO: true, bAr: {bAz: true}}, (key, value) => [key.toLowerCase(), value], {deep: true});
 //=> {foo: true, bar: {baz: true}}
+
+const newObject = mapObject({one: 1, two: 2}, (key, value) => value === 1 ? [key, value] : mapObject.mapObjectSkip);
+//=> {one: 1}
 ```
 
 ## API
@@ -35,7 +38,7 @@ Source object to copy properties from.
 
 #### mapper
 
-Type: `(sourceKey, sourceValue, source) => [targetKey, targetValue, mapperOptions?]`
+Type: `(sourceKey, sourceValue, source) => [targetKey, targetValue, mapperOptions?] | mapObject.mapObjectSkip`
 
 Mapping function.
 
@@ -66,6 +69,21 @@ Default: `{}`
 
 Target object to map properties on to.
 
+### mapObject.mapObjectSkip
+
+Return this value from a `mapper` function to exclude the key from the new object.
+
+```js
+const mapObject = require('map-obj');
+
+const object = {one: 1, two: 2}
+const mapper = (key, value) => value === 1 ? [key, value] : mapObject.mapObjectSkip
+const result = mapObject(object, mapper);
+
+console.log(result);
+//=> {one: 1}
+```
+
 ## Related
 
 - [filter-obj](https://github.com/sindresorhus/filter-obj) - Filter object keys and values into a new object

test.js

@@ -163,3 +163,18 @@ test('__proto__ keys are safely dropped', t => {
 	// the prototype by identity
 	t.is(Object.getPrototypeOf(output), Object.prototype);
 });
+
+test('remove keys (#36)', t => {
+	const object = {
+		one: 1,
+		two: 2
+	};
+
+	const expected = {
+		one: 1
+	};
+
+	const mapper = (key, value) => value === 1 ? [key, value] : mapObject.mapObjectSkip;
+	const actual = mapObject(object, mapper, {deep: true});
+	t.deepEqual(actual, expected);
+});