简单的 Promise 实现

参考 http://www.tuicool.com/articles/RzQRV3

var PENDING = undefined,
    FULLFILLED = 1,
    REJECTED = 2;
var Promise = function(resolver) {

    var promise = this;
    this.value;
    this.reason;
    this.status = PENDING;
    this.resolver;
    this.rejecter;

    var resolve = function(value) {
        promise.value = value;
        promise.status = FULLFILLED;
        if (promise.resolver || promise.rejecter) {
            var ret = promise.resolver.call(null, promise.value);
            if(ret instanceof Promise) {
                ret.then(promise.resolver_relsolve);
            } else {
                promise.resolver_relsolve(promise.value);
            }
        }
    }

    var reject = function(err) {
        promise.value = err;
        promise.status = REJECTED;
        if (promise.resolver || promise.rejecter) {
            var ret = promise.rejecter.call(null, promise.value);
            if(ret instanceof Promise) {
                ret.then(function(){}, promise.rejecter_reject);
            } else {
                promise.rejecter_reject(promise.value);
            }
        }
    }
    
    resolver(resolve, reject);
}

// Promise then 方法
Promise.prototype.then = function(onFulfilled, onRejected) {
    var promise = this;

    if (promise.status == FULLFILLED) {
        var ret = onFulfilled(promise.value);

        if(ret instanceof Promise) {
            return ret;
        } else {
            return new Promise(function(resolve) {
                resolve(ret);
            });
        }

    } else if (promise.status == REJECTED) {
        var ret = onRejected(promise.value);

        if(ret instanceof Promise) {
            return ret;
        } else {
            return new Promise(function(resolve, reject) {
                reject(ret);
            });
        }    
    }

    return new Promise(function(resolve, reject) {
        promise.resolver = onFulfilled;
        promise.resolver_relsolve = resolve;
        promise.rejecter = onRejected;
        promise.rejecter_reject = reject;
    });
}

测试

var p = new Promise(function(resolve, reject) {
    console.log('before');
    setTimeout(function() {
        resolve(10);
    }, 2000);
});

p.then(function() {
    console.log('after', arguments);
}).then(function() {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve(20)
        }, 2000);
    });
}).then(function() {
    console.log(arguments);
});

输出

// 两秒
before
after [10]
// 两秒
[20]