SOURCE

function promise(constructor) {
    this.status = "pending";
    this.value = undefined;
    this.reason = undefined;

    this.resolveArray = [];
    this.rejectArray = [];
    self = this;
    function resolve(value) {
        if(this.status === "pending") {
            self.value = value;
            self.status = "resolved";
            self.resolveArray.forEach(item => {
                item(self.vlaue);
            })
        }
    };
    function reject(reason) {
        if(this.status === "pending") {
            self.reason = reason;
            self.status = "rejected";
            self.rejectArray.forEach(item => {
                item(self.reason)
            })
        }
    };

    try{
        constructor(resolve, reject);
    }catch(error) {
        console.log(error);
    }
}

promise.prototype.then = function(onFulfilledill, onRejected) {
    onFulfilledill = typeof onFulfilledill==="function" ? onFulfilledill : value => value;
    onRejected = typeof onRejected ==="function" ? onRejected : error => {throw error};


    let pro = new promise((resolve, reject) =>{
        if(this.status === "pending") {
            this.resolveArray.push(onFulfilledill(this.value));
            this.rejectArray.push(onRejected(this.value));
        }
        if(this.status === "resolved") {
            onFulfilledill(this.value);
        }
        if(this.status === "rejected") {
            onRejected(this.reason);
        }
    })
    return pro;
}