Good job thus far. Here's some feedback

• GET /posts: all good!
• GET /posts/:postId : this is pretty good. When you design a REST API route which should return a specific resource (here you want to return a single post), it's more usual to return the object. Right now, this route will return an array (with a single object).
• Right now, the delete routes don't delete the element from the array. For example, if you try to delete an item and then go to GET /posts, you still get all the posts (even the deleted ones). In order to delete one post, you should remove it from the array. For instance, you could create a new array without the post you want to delete, and assign this value to the posts variable. Look at the Array documentation to see which method you could use: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array.