Why do we need asynchronous programming

JavaScript is a single threaded language, and time-consuming operations such as network requests and file read/write must be executed asynchronously, otherwise the interface will freeze.

The problem of callback hell

Multi layer nested callback code is difficult to maintain.

Promise Three States

  • pending: In progress
  • fulfilled: Success
  • rejected: Failed
fetch('/api/user').then(res => res.json()).then(data => console.log(data)).catch(err => console.error(err));

Asynchronous/Await syntax sugar

ES2017 introduces asynchronous code that looks like synchronization:

async function loadUser() {

parallel execution

Multiple independent requests are parallelized using Promise.all.

Common Pitfalls

  • Forgetting await results in obtaining Promise object instead of result
  • Incorrect use of await in loop leads to serial execution
  • Missing try/catch leads to uncaught exceptions

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