feat: add assign id function (#10)

Author: bahmutov

README.md

@@ -91,6 +91,17 @@ Typically, the backend creates new `id` for each `POST /resource` call that does
 { rest: { assignId: true, todos: 'todos.json' } }
 ```
 
+You can also pass your own synchronous function to return an id
+
+```js
+{
+  rest: {
+    assignId: () => '123-abc',
+    todos: 'todos.json',
+  },
+}
+```
+
 ## Small print
 
 Author: Gleb Bahmutov <gleb.bahmutov@gmail.com> © 2025

cypress/e2e/server-id-function.cy.js

@@ -0,0 +1,30 @@
+/// <reference types="cypress" />
+
+chai.config.truncateThreshold = 0
+
+describe(
+  'Todos with server-side ID',
+  {
+    rest: {
+      assignId: () => '123-abc',
+      todos: 'todos.json',
+    },
+  },
+  () => {
+    beforeEach(() => {
+      cy.visit('app-server-id/index.html')
+      cy.contains('h1', 'Server ID')
+    })
+
+    it('uses custom stub to create server id', () => {
+      cy.get('input.new-todo').type('Buy milk{enter}')
+
+      cy.step('Check the created UUID')
+      cy.contains('li.todo', 'Buy milk').should(
+        'have.attr',
+        'data-todo-id',
+        '123-abc',
+      )
+    })
+  },
+)

src/index.js

@@ -47,7 +47,13 @@ beforeEach(function prepareRestApi() {
         const item = structuredClone(req.body)
         // modify the id?
         if (assignId && !('id' in item)) {
-          item.id = crypto.randomUUID()
+          if (assignId === true) {
+            item.id = crypto.randomUUID()
+          } else if (typeof assignId === 'function') {
+            item.id = assignId()
+          } else {
+            throw new Error('assignId has invalid value')
+          }
         }
         data.push(item)
         req.reply(201, item)