Nodejs 在后台运行 function

Question

I've nodes program which I need to run two function in the beginning of the program And later on access the function results, currently with await each function at a time this works, However in order to save a time and not waiting to GetService and GetProcess as I need the data later on in the project It takes about 4 seconds to get this data and I want to run it on the background as I don't need the results immediately, How I can do it in node js, If I run promise.all It would wait until the getService and getProcess and then go to rest of the program.

an example

function main() {

//I want to run this both function in background to save time
let service = await GetServices();
this.process = await GetProcess();


…..//Here additional code is running 

//let say that after 30 second this code is called
 Let users = GetUser(service);

 Let users = GetAdress(this.process);
} 

im actually running yeoman generator https://yeoman.io/authoring/ https://yeoman.io/authoring/user-interactions.html

export default class myGenerator extends Generator {

//here I want run those function in background to save time as the prompt to the user takes some time (lets say user have many questions...) 
async initializing() {
    let service = await GetServices();
    this.process = await GetProcess();
}

async prompting() {
    const answers = await this.prompt([
      {
        type: "input",
        name: "name",
        message: "Your project name",
        default: this.appname // Default to current folder name
      },
      {
        type: "confirm",
        name: "list",
        choises: this.process //here I need to data from the function running in background
      }
    ]);

}

Answer 1

Let's assume that getServices() may take 3 seconds and getProcess() may take 4 seconds, so if you run these both functions at the same time you will be returned in total 4 seconds with the return values from both promises.

You can execute the code while this process is running in the background there will be a callback when the promises resolved, your late functions will be called at this stage.

Check the below simple example;

 let service; let process; function main() { // Both functions will execute in background Promise.all([getServices(), getProcess()]).then((val) => { service = val[0]; process = val[1]; console.log(service, process); // Aafter completed this code will be called // let users = GetUser(service); // let users = GetAdress(process); console.log('I am called after all promises completed.') }); // Current example. // let service = await GetServices(); // this.process = await GetProcess(); /* Code blocks.. */ console.log('Code will execute without delay...') } function getServices() { return new Promise((resolve, reject) => { setTimeout(() => { resolve("service is returned") }, 3000); }); } function getProcess() { return new Promise((resolve, reject) => { setTimeout(() => { resolve("process is returned") }, 4000); }); } main();

Answer 2

You can start the asynchronous operation but not await it yet:

function suppressUnhandledRejections(p) {
  p.catch(() => {});
  return p;
}

async function main() {
  // We have to suppress unhandled rejections on these promises. If they become
  // rejected before we await them later, we'd get a warning otherwise.
  const servicePromise = suppressUnhandledRejections(GetServices());
  this.processPromise = suppressUnhandledRejections(GetProcess());

  // Do other stuff
  const service = await servicePromise;
  const process = await this.processPromise;
}

Also consider using Promise.all() which returns a promise for the completion of all promises passed to it.

async function main() {
  const [ services, process, somethingElse ] = await Promise.all([
    GetServices(),
    GetProcess(),
    SomeOtherAsyncOperation(),
  ]);

  // Use the results.
}

Answer 3

To do what who you need, you have to understand the event loop .

Nodejs is designed to work in a single thread unlike languages like go, however nodejs handle proccess on different threads. so you can use nextTick () to add a new event to the main thread and it will be executed at the end of the whole block.

    function main() {

    //I want to run this both function in background to save time
    let service = await GetServices();
    this.process = await GetProcess();


    …..//Here additional code is running 

    //Let say that after 30 second this code is called
     Let users = GetUser(service);

     Let users = GetAdr(this.process);
    } 


 function someFunction(){
 // do something...
  }
 main();
 process.nextTick(someFunction());// happens after all main () processes are terminated...