feat(collections): implement associateBy (#1071)

Author: LionC

collections/associate_by.ts

@@ -0,0 +1,40 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+/**
+  * Transforms the given array into a Record, extracting the key of each element using the given selector.
+  * If the selector produces the same key for multiple elements, the latest one will be used (overriding the
+  * ones before it).
+  *
+  * Example:
+  *
+  * ```ts
+  * import { associateBy } from "./associate_by.ts"
+  *
+  * const users = [
+  *     { id: 'a2e', userName: 'Anna' },
+  *     { id: '5f8', userName: 'Arnold' },
+  *     { id: 'd2c', userName: 'Kim' },
+  * ]
+  * const usersById = associateBy(users, it => it.id)
+  *
+  * console.assert(usersById === {
+  *     'a2e': { id: 'a2e', userName: 'Anna' },
+  *     '5f8': { id: '5f8', userName: 'Arnold' },
+  *     'd2c': { id: 'd2c', userName: 'Kim' },
+  * })
+  * ```
+  */
+export function associateBy<T>(
+  array: Array<T>,
+  selector: (el: T) => string,
+): Record<string, T> {
+  const ret: Record<string, T> = {};
+
+  for (const element of array) {
+    const selectedValue = selector(element);
+
+    ret[selectedValue] = element;
+  }
+
+  return ret;
+}

collections/associate_by_test.ts

@@ -0,0 +1,125 @@
+// Copyright 2018-2021 the Deno authors. All rights reserved. MIT license.
+
+import { assertEquals } from "../testing/asserts.ts";
+import { associateBy } from "./associate_by.ts";
+
+function associateByTest<T>(
+  input: [Array<T>, (el: T) => string],
+  expected: { [x: string]: T },
+  message?: string,
+) {
+  const actual = associateBy(...input);
+  assertEquals(actual, expected, message);
+}
+
+Deno.test({
+  name: "[collections/associateBy] no mutation",
+  fn() {
+    const arrayA = ["Foo", "Bar"];
+    associateBy(arrayA, (it) => it.charAt(0));
+
+    assertEquals(arrayA, ["Foo", "Bar"]);
+  },
+});
+
+Deno.test({
+  name: "[collections/associateBy] empty input",
+  fn() {
+    associateByTest(
+      [[], () => "a"],
+      {},
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/associateBy] constant key",
+  fn() {
+    associateByTest(
+      [[1, 3, 5, 6], () => "a"],
+      { a: 6 },
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/associateBy] empty key",
+  fn() {
+    associateByTest(
+      [
+        ["Foo", "b"],
+        (it) => it.charAt(1),
+      ],
+      {
+        "o": "Foo",
+        "": "b",
+      },
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/associateBy] duplicate keys",
+  fn() {
+    associateByTest(
+      [
+        ["Anna", "Marija", "Karl", "Arnold", "Martha"],
+        (it) => it.charAt(0),
+      ],
+      {
+        "A": "Arnold",
+        "M": "Martha",
+        "K": "Karl",
+      },
+    );
+    associateByTest(
+      [
+        [1.2, 2, 2.3, 6.3, 6.9, 6],
+        (it) => Math.floor(it).toString(),
+      ],
+      {
+        "1": 1.2,
+        "2": 2.3,
+        "6": 6,
+      },
+    );
+  },
+});
+
+Deno.test({
+  name: "[collections/associateBy] associates",
+  fn() {
+    associateByTest(
+      [
+        [
+          { name: "test", done: false },
+          { name: "build", done: true },
+          { name: "deploy", done: false },
+          { name: "audit", done: true },
+        ],
+        (it) => it.name,
+      ],
+      {
+        "test": { name: "test", done: false },
+        "build": { name: "build", done: true },
+        "deploy": { name: "deploy", done: false },
+        "audit": { name: "audit", done: true },
+      },
+    );
+    associateByTest(
+      [
+        [
+          "[email protected]",
+          "[email protected]",
+          "[email protected]",
+        ],
+        (it) => it.substring(0, it.indexOf("@")),
+      ],
+      {
+        "anna": "[email protected]",
+        "josh": "[email protected]",
+        "kim": "[email protected]",
+      },
+    );
+  },
+});