(async () => { })(); what is this?

Question

async function test() {
  (async () => {            
    var a = await this.test1();
    var b = await this.test2(a);
    var c = await this.test3(b);  
    this.doThis(a,b,c);                              
  })();
}

What does it mean to put methods (test1,test2,test3) inside async () => {})()? I find it faster than

async function test() {          
  var a = await this.test1();
  var b = await this.test2(a);
  var c = await this.test3(b);  
  this.doThis(a,b,c); 
}

Any downside of using it?

Answer 1

Both return a promise but they return different promises.

The first will return a promise that may resolve before this.test1()'s result resolves.

The second returns a promise that only resolves after the final call to this.doThis(a,b,c);.

This has been called the "fire and forget pattern":

Often in application development you want a process to call another thread and continue the process flow, without waiting for a response from the called thread. This pattern is called the “fire and forget” pattern.

You can see this in

function logEventually(str) {
  return new Promise((resolve) => {
    setTimeout(() => {
      console.log(str);
      resolve(null);
    }, 0);
  });
}

async function a() {
  await logEventually('in a 1');
  await logEventually('in a 2');
  await logEventually('in a 3');
  return await logEventually('end of a');
}

async function b() {
  (async () => {
    await logEventually('in b 1');
    await logEventually('in b 2');
    await logEventually('in b 3');
  })();
  return await logEventually('end of b');
}

a();
b();