feat(collection): add mapNotNullish (#1103)

collections/map_not_nullish.ts

@@ -0,0 +1,39 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+/**
+ * Returns a new array, containing all elements in the given array transformed using the given transformer, except the ones
+ * that were transformed to `null` or `undefined`
+ *
+ * Example:
+ *
+ * ```typescript
+ * import { mapNotNullish } from "./map_not_nullish.ts";
+ * import { assertEquals } from "../testing/asserts.ts";
+ *
+ * const people = [
+ *     { middleName: null },
+ *     { middleName: 'William' },
+ *     { middleName: undefined },
+ *     { middleName: 'Martha' },
+ * ]
+ * const foundMiddleNames = mapNotNullish(people, it => it.middleName)
+ *
+ * assertEquals(foundMiddleNames, [ 'William', 'Martha' ])
+ * ```
+ */
+export function mapNotNullish<T, O>(
+  array: Array<T>,
+  transformer: (el: T) => O,
+): Array<NonNullable<O>> {
+  const ret = new Array<NonNullable<O>>();
+
+  for (const element of array) {
+    const transformedElement = transformer(element);
+
+    if (transformedElement !== undefined && transformedElement !== null) {
+      ret.push(transformedElement as NonNullable<O>);
+    }
+  }
+
+  return ret;
+}

collections/map_not_nullish_test.ts

@@ -0,0 +1,92 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+import { assertEquals } from "../testing/asserts.ts";
+import { mapNotNullish } from "./map_not_nullish.ts";
+
+function mapNotNullishTest<T, O>(
+  input: [Array<T>, (el: T) => O | undefined | null],
+  expected: Array<O>,
+  message?: string,
+) {
+  const actual = mapNotNullish(...input);
+  assertEquals(actual, expected, message);
+}
+
+Deno.test({
+  name: "[collections/mapNotNullish] no mutation",
+  fn() {
+    const array = [1, 2, 3, 4];
+    mapNotNullish(array, (it) => it * 2);
+
+    assertEquals(array, [1, 2, 3, 4]);
+  },
+});
+
+Deno.test({
+  name: "[collections/mapNotNullish] empty input",
+  fn() {
+    mapNotNullishTest(
+      [[], (it) => it],
+      [],
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/mapNotNullish] identity",
+  fn() {
+    mapNotNullishTest(
+      [
+        [[], 1, 3],
+        (it) => it,
+      ],
+      [[], 1, 3],
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/mapNotNullish] mappers without nullish values",
+  fn() {
+    mapNotNullishTest(
+      [
+        ["Anna", "Kim", "Hans"],
+        (it) => it.charAt(0),
+      ],
+      ["A", "K", "H"],
+    );
+    mapNotNullishTest(
+      [
+        [3, 4, 5, 6],
+        (it) => it * 2,
+      ],
+      [6, 8, 10, 12],
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/mapNotNullish] mappers with nullish values",
+  fn() {
+    mapNotNullishTest(
+      [
+        ["Errors: 5", "Success", "Warnings: 10", "..."],
+        (it) =>
+          it.match(/\w+: (?<numberOfProblems>\d+)/u)?.groups?.numberOfProblems,
+      ],
+      ["5", "10"],
+    );
+    mapNotNullishTest(
+      [
+        [
+          { first: "Kim", middle: undefined, last: "Example" },
+          { first: "Arthur", middle: "Hans", last: "Somename" },
+          { first: "Laura", middle: "Marija", last: "Anothername" },
+          { first: "Sam", middle: null, last: "Smith" },
+        ],
+        (it) => it.middle,
+      ],
+      ["Hans", "Marija"],
+    );
+  },
+});