Handwriting promise

const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
function Promise(executor) {
    
    var _this = this
    this.state = PENDING; // state 
    this.value = undefined; // Successful results 
    this.reason = undefined; // Reasons for failure 

    this.onFulfilled = [];// Successful callback 
    this.onRejected = []; // Failed callback 
    function resolve(value) {
    
        if(_this.state === PENDING){
    
            _this.state = FULFILLED
            _this.value = value
            _this.onFulfilled.forEach(fn => fn(value))
        }
    }
    function reject(reason) {
    
        if(_this.state === PENDING){
    
            _this.state = REJECTED
            _this.reason = reason
            _this.onRejected.forEach(fn => fn(reason))
        }
    }
    try {
    
        executor(resolve, reject);
    } catch (e) {
    
        reject(e);
    }
}
Promise.prototype.then = function (onFulfilled, onRejected) {
    
    if(this.state === FULFILLED){
    
        typeof onFulfilled === 'function' && onFulfilled(this.value)
    }
    if(this.state === REJECTED){
    
        typeof onRejected === 'function' && onRejected(this.reason)
    }
    if(this.state === PENDING){
    
        typeof onFulfilled === 'function' && this.onFulfilled.push(onFulfilled)
        typeof onRejected === 'function' && this.onRejected.push(onRejected)
    }
}