Prompt Title: useful built-in APIs and classes JS

Sure, there are many more useful built-in JavaScript APIs and classes. Here are some of them:

1. Fetch API: The Fetch API provides an interface for fetching resources (including across the network). It's more powerful and flexible than the older XMLHttpRequest.

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   fetch('https://api.example.com/data', { method: 'GET' })
   .then(response => response.json())
   .then(data => console.log(data))
   .catch((error) => console.error('Error:', error));
   

2. Promise: A Promise is an object representing the eventual completion or failure of an asynchronous operation. It's used for asynchronous computations and can help to avoid callback hell.

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   let promise = new Promise(function(resolve, reject) {
       // Do something, and call resolve if successful, reject if not
   });
   promise.then(value => console.log("Success: ", value)).catch(error => console.log("Failed: ", error));
   

3. Array: Array is a global object that is used in the construction of arrays. It has many useful methods like map(), reduce(), filter(), forEach(), sort(), etc.

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   let array = [1, 2, 3, 4, 5];
   let squares = array.map(x => x * x);
   console.log(squares); // [1, 4, 9, 16, 25]
   

4. Math: The Math object has properties and methods for mathematical constants and functions.

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   console.log("Pi is " + Math.PI);
   console.log("The square root of 16 is " + Math.sqrt(16));
   

5. Set: The Set object lets you store unique values of any type, whether primitive values or object references.

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   let mySet = new Set();
   mySet.add(1);
   mySet.add('some text');
   mySet.add({a: 1, b: 2});
   console.log(mySet.size); // 3
   

6. Map: The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value may be used as either a key or a value.

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   let myMap = new Map();
   myMap.set('name', 'John');
   myMap.set('age', 30);
   console.log(myMap.get('name')); // John
   console.log(myMap.size); // 2
   

7. Date: The Date object represents a date and time functionality in JavaScript. It allows you to get the current date and time, store a date in a variable, and perform all sorts of operations with dates.

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   let now = new Date();
   console.log(now.toString());
   

These are just some of the many built-in APIs and classes in JavaScript. Each has its own use cases and are extremely useful for different scenarios.