原生JavaScript实现一个简单的Promise构造函数示例

下面demo示例，只支持实例的then和catch,代码如下：

function PromiseDiffer(fn){
    var self = this;
    this.status = 'pendding';
    this.value = undefined;
    this.reason = null;
    function resolve(data) {
        self.status = 'fulfilled';
        self.successFn && self.successFn(data);
        self.value = data;
    }
    function reject(error) {
        self.status = 'rejected';
        self.errorFn && self.errorFn(error);
        self.reason = error;
    }

    fn && fn(resolve,reject);

    PromiseDiffer.prototype.then = function(onFulfilledCallback,onRejectedCallback) {
        if(this.status == 'fulfilled') {
            onFulfilledCallback && onFulfilledCallback(this.value);
        }
        if(this.status == 'rejected') {
            onRejectedCallback && onRejectedCallback(this.reason);
        }
        this.successFn = onFulfilledCallback;
        this.errorFn = onRejectedCallback;
        return this;
    }

    PromiseDiffer.prototype.catch = function(onRejected) {
        return this.then(null, onRejected);
    }

}

调用方式跟Promise一样，如下：

var promise = new PromiseDiffer(function(resolve,reject){
    resolve('1');
})
promise.then(function(data){
    console.log(data)
}).catch(function(err){

})

后续再添加完善其他方法。