How to check if a Promise is pending [duplicate]

问题

This question already has an answer here:

• How can I synchronously determine a JavaScript Promise's state? 20 answers

I have this situation in which I would like to know what the status is of a promise. Below, the function start only calls someTest if it is not running anymore (Promise is not pending). The start function can be called many times, but if its called while the tests are still running, its not going to wait and returns just false

class RunTest {
    start() {
         retVal = false;

         if (!this.promise) {
             this.promise = this.someTest();
             retVal = true;                
         }

         if ( /* if promise is resolved/rejected or not pending */ ) {
             this.promise = this.someTest();
             retVal = true;
         }

         return retVal;
    }

    someTest() {
        return new Promise((resolve, reject) => {
            // some tests go inhere
        });
    }
}

I cannot find a way to simply check the status of a promise. Something like this.promise.isPending would be nice :) Any help would be appreciated!

回答1:

You can attach a then handler that sets a done flag on the promise (or the RunTest instance if you prefer), and test that:

     if (!this.promise) {
         this.promise = this.someTest();
         this.promise.catch(() => {}).then(() => { this.promise.done = true; });
         retVal = true;                
     }

     if ( this.promise.done ) {
         this.promise = this.someTest();
         this.promise.catch(() => {}).then(() => { this.promise.done = true; });
         retVal = true;
     }

Notice the empty catch() handler, it's crucial in order to have the handler called regardless of the outcome of the promise. You probably want to wrap that in a function though to keep the code DRY.

回答2:

class RunTest {
   constructor() {
    this.isRunning = false;
   }
   start() {
      console.log('isrunning', this.isRunning);
      var retVal = false;
      if(!this.isRunning) {
        this.promise = this.someTest();
        this.promise.catch().then(() => { this.isRunning = false; });
        retVal = true;                
      }
      return retVal;
    }
    someTest() {
        this.isRunning = true;
        return new Promise((resolve, reject) => {
          setTimeout(function() {
             //some tests go inhere
             resolve();
           }, 1000);
        });
    }
};

var x = new RunTest();

x.start(); //logs false
x.start(); //logs true

setTimeout(function() {
    //wait for a bit
  x.start(); //logs false
}, 2000);

来源：https://stackoverflow.com/questions/36294109/how-to-check-if-a-promise-is-pending