简单的Promise

function MYPromise(fn) {
    var self = this
    self.onResolvedCallback = []
    function resolve(value) {
        setTimeout(() => {
            self.data = value
            self.onResolvedCallback.forEach(callback => callback(value))
        })
    }
    fn(resolve)
}
MYPromise.prototype.then = function (fn) {
    var self = this
    return new MYPromise(resolve => {
        self.onResolvedCallback.push(function () {
            var result = fn(self.data)
            if (result instanceof MYPromise) {
                result.then(resolve)
            } else {
                resolve(result)
            }
        })
    })
}

　　

参考文章：https://juejin.im/post/5e6f4579f265da576429a907#heading-0