javascript - How to delay excuting the code inside Promise? -


my code:

class mypromise extends promise {    constructor(func) {      super(func);    }    somepromiseextensionmethod() {      // stuff...    }  }    let test = async function() {    let promise = new mypromise(function (resolve) {      console.log('foo');        resolve();    });        console.log('start printing:');        await promise;  };    test();

i'm leaning promise , want create class derived promise class.

my question: there way delay printing foo until call await promise?

update:

i want create class because want run methods while object promise. (just like: mypromise.somepromiseextensionmethod()).

update 2: real project:

    let task = (function () {          let __data = {};            __data.run = function (...args) {              if (args.length) {                  let func = args[0];                    return new task(func);              }          };                    class task extends promise {              constructor(action) {                  super(action);              }              static set run(value) {                  return __data.run;              }              static run() {                  return (...args) => __data.run.call(this, ...args);              }          }          return task;      }());            let test = async () => {        let task = task.run(r => r(console.log('foo')));                console.log('start printing');                await task;      };            test();

is there way delay printing foo until call await promise?

yes, known "lazy" promises. notice await sugar calling then method, can overwriting then method , run executor callback:

class mypromise extends promise {   constructor(func) {     let run;     super((resolve, reject) => {       run = () => func(resolve, reject);     });     this.run = run;   }   then(onfulfilled, onrejected) {     const run = this.run;     if (run) {       this.run = null;       run();     }     return super.then(onfulfilled, onrejected);   } }  (async function test() {   let promise = new mypromise(function (resolve) {     console.log('foo');     resolve();   });    console.log('start printing:');    await promise; }());

please use learning exercise, wouldn't recommend use pattern in actual project.


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