参考答案

参考回答：

function Promise(fn) {
var state = 'pending',
value = null,
callbacks = [];
this.then = function (onFulfilled, onRejected) {
return new Promise(function (resolve, reject) {
handle({
onFulfilled: onFulfilled || null,
onRejected: onRejected || null,
resolve: resolve,
reject: reject
});
});
};
function handle(callback) {
if (state === 'pending') {
callbacks.push(callback);
return;
}
var cb = state === 'fulfilled' ? callback.onFulfilled : callback.onRejected,
ret;
if (cb === null) {
cb = state === 'fulfilled' ? callback.resolve : callback.reject;
cb(value);
return;
}
ret = cb(value);
callback.resolve(ret);
}
function resolve(newValue) {
if (newValue && (typeof newValue === 'object' || typeof newValue === 'function')) {
var then = newValue.then;
if (typeof then === 'function') {
then.call(newValue, resolve, reject);
return;
}
}
state = 'fulfilled';
value = newValue;
execute();
}
function reject(reason) {
state = 'rejected';
value = reason;
execute();
}
function execute() {
setTimeout(function () {
callbacks.forEach(function (callback) {
handle(callback);
});
}, 0);
}
fn(resolve, reject);
}