Add byIndex helper

README.md

@@ -69,6 +69,7 @@ export default {
 | count | `count('arrayProperty')` | no |
 | countBy | `countBy('arrayProperty', 'done', true)` | no |
 | classObject | `classObject('isPrimary', 'has-title:title', 'wide')` | yes |
+| byIndex | `byIndex('arrayProperty', 0)` | no |
 
 `x` means that it can be either value or property name. If you provide a string and there will be a property with that name it's value will be used to perform the check.
 

lib/index.js

@@ -209,3 +209,11 @@ export function classObject(...args) {
     }, {});
   }
 }
+
+export function byIndex(arg, index) {
+  return function() {
+    const isArray = Array.isArray(this[arg])
+    console.assert(isArray, 'computed helper "byIndex" requires property of array type')
+    return isArray ? this[arg][index] : undefined
+  }
+}

tests/byIndex.spec.js

@@ -0,0 +1,37 @@
+import * as computed from '../lib/index'
+
+global.console.assert = jest.fn()
+
+describe('byIndex', () => {
+  const context = {
+    todos: [{
+      id: 1,
+      done: false
+    }, {
+      id: 2,
+      done: true
+    }, {
+      id: 3,
+      done: true
+    }],
+    arr: [1, 2, 3, 4],
+    items: 'test'
+  }
+
+  it('finds an item in array by index', () => {
+    expect(
+      computed.byIndex('todos', 2).bind(context)()
+    ).toEqual({ id: 3, done: true })
+
+    expect(
+      computed.byIndex('arr', 0).bind(context)()
+    ).toEqual(1)
+  })
+
+  it('returns an undefined value when the argument is not an array', () => {
+    expect(
+      computed.byIndex('items', 0).bind(context)()
+    ).toEqual(undefined)
+    expect(console.assert.mock.calls.slice(-1)[0][0]).toBe(false)
+  })
+})