v.js

class MyPromise{
    constructor(executor) {
        // The initial state of the Promise object should be pending
        this.status = "pending"; 
      
        // Store the value after success or reason after fail
        this.value = undefined;      
        
        // Define a resolve function
        let resolve = result => {
            // If the state of a Promise instance has changed before, it should not be changed again.
            if(this.status !== "pending") return;
          
            this.status = "resolved";
            this.value = result;
        }
        
        // Define a resolve function
        let reject = reason => {
            // If the state of a Promise instance has changed before, it should not be changed again.
            if(this.status !== "pending") return;
            this.status = "rejected";
            this.value = reason;
        }
        
        // Even if an exception occurs to the executor function, 
        //   it should be handled within the Promise function,
        //   not thrown outside the Promise function.
        try {
            executor(resolve, reject)
        } catch(err) {
            reject(err)
        }
    }
}