Add packages/common/src/indexBy.ts

packages/common/jest.config.js

@@ -0,0 +1,17 @@
+module.exports = {
+  roots: [
+    '<rootDir>/src'
+  ],
+  transform: {
+    '^.+\\.tsx?$': 'ts-jest'
+  },
+  testRegex: '(/__tests__/.*|\\.(test|spec))\\.tsx?$',
+  moduleFileExtensions: [
+    'ts',
+    'tsx',
+    'js',
+    'jsx'
+  ],
+  setupFiles: ['<rootDir>/jest.setup.js'],
+  verbose: false
+}

packages/common/src/indexBy.test.ts

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+import {indexBy} from './indexBy'
+
+describe('indexBy', () => {
+
+  const items = [{
+    id: 10,
+    name: 'one',
+    value: true,
+  }, {
+    id: 20,
+    name: 'two',
+    value: false,
+  }]
+
+  it('indexes objects by string property', () => {
+    expect(indexBy(items, 'id')).toEqual({
+      10: items[0],
+      20: items[1],
+    })
+  })
+
+  it('indexes objects by numeric property', () => {
+    expect(indexBy(items, 'name')).toEqual({
+      one: items[0],
+      two: items[1],
+    })
+  })
+})

packages/common/src/indexBy.ts

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+export function indexBy<T extends object, K extends keyof T>(
+  items: T[],
+  key: T[K] extends string | number ? K : never,
+) {
+  return items.reduce((obj, item) => {
+    obj[String(item[key])] = item
+    return obj
+  }, {} as {[k: string]: T})
+}