What does the following code output? Why?

const token = (async () => {
  const token = await new Promise(resolve => {
    setTimeout(() => resolve("abc"), 2000);
  });
  console.log("1", token);
  return token;
})();

console.log("2", token);
Javascript
May.19,2022

2 and then 1

The token in

2 is a variable assignment, so you don't have to wait for the asynchronous execution to finish
because 1 contains token and token is in the scope of the asynchronous function, so you need to finish the asynchronous code execution before you get the result

.

Open chrome, press F12, type the code into the console and press enter 


const token = (async () => {
  const token = await new Promise(resolve => {
    setTimeout(() => resolve("abc"), 2000);
  });
  console.log("1", token);  // 1  abc
  return token;  //   async return Promise
})();

console.log("2", token);  // 2 Promise

in async function, return await is useless. Because async function always contains the return value Promise.resolve, return await doesn't actually do anything unless you add extra time before the overall Promise parses or rejects. This pattern is almost certainly because programmers ignore the return semantics of async function.

Previous: How to split two arrays based on js

Next: Is there a desktop framework for go + WebAssembly + [chrome / electron]?

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
© 2021 CodesHelper
Advertising Contact us About us
Menu