Promise是一种异步编程的解决方案，它可以避免回调地狱，使得异步操作更加简洁和易于维护。下面是Promise的源码实现原理：
- Promise构造函数
Promise构造函数接受一个executor函数作为参数，executor函数会立即执行，并传入两个参数resolve和reject。resolve函数用于将Promise状态从pending变为fulfilled，reject函数用于将Promise状态从pending变为rejected。
function Promise(executor) {
var self = this;
self.status = 'pending'; // Promise状态
self.data = undefined; // Promise的值
self.onResolvedCallback = []; // Promise resolve时的回调函数集合
self.onRejectedCallback = []; // Promise reject时的回调函数集合
function resolve(value) {
if (self.status === 'pending') {
self.status = 'fulfilled';
self.data = value;
for (var i = 0; i < self.onResolvedCallback.length; i++) {
self.onResolvedCallback[i](value);
}
}
}
function reject(reason) {
if (self.status === 'pending') {
self.status = 'rejected';
self.data = reason;
for (var i = 0; i < self.onRejectedCallback.length; i++) {
self.onRejectedCallback[i](reason);
}
}
}
try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
- Promise.prototype.then方法
Promise.prototype.then方法用于注册Promise状态变为fulfilled或rejected时的回调函数。它接受两个参数，onResolved和onRejected，分别表示Promise状态变为fulfilled或rejected时的回调函数。
Promise.prototype.then = function(onResolved, onRejected) {
var self = this;
var promise2;
onResolved = typeof onResolved === 'function' ? onResolved : function(value) { return value; };
onRejected = typeof onRejected === 'function' ? onRejected : function(reason) { throw reason; };
if (self.status === 'fulfilled') {
return promise2 = new Promise(function(resolve, reject) {
try {
var x = onResolved(self.data);
if (x instanceof Promise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (e) {
reject(e);
}
});
}
if (self.status === 'rejected') {
return promise2 = new Promise(function(resolve, reject) {
try {
var x = onRejected(self.data);
if (x instanceof Promise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (e) {
reject(e);
}
});
}
if (self.status === 'pending') {
return promise2 = new Promise(function(resolve, reject) {
self.onResolvedCallback.push(function(value) {
try {
var x = onResolved(value);
if (x instanceof Promise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (e) {
reject(e);
}
});
self.onRejectedCallback.push(function(reason) {
try {
var x = onRejected(reason);
if (x instanceof Promise) {
x.then(resolve, reject);
} else {
resolve(x);
}
} catch (e) {
reject(e);
}
});
});
}
};
- Promise.prototype.catch方法
Promise.prototype.catch方法用于注册Promise状态变为rejected时的回调函数。它接受一个参数onRejected，表示Promise状态变为rejected时的回调函数。
Promise.prototype.catch = function(onRejected) {
return this.then(null, onRejected);
};
- Promise.resolve方法
Promise.resolve方法用于创建一个状态为fulfilled的Promise对象，它接受一个参数value，表示Promise的值。
Promise.resolve = function(value) {
return new Promise(function(resolve, reject) {
resolve(value);
});
};
- Promise.reject方法
Promise.reject方法用于创建一个状态为rejected的Promise对象，它接受一个参数reason，表示Promise被拒绝的原因。
Promise.reject = function(reason) {
return new Promise(function(resolve, reject) {
reject(reason);
});
};
- Promise.all方法
Promise.all方法用于将多个Promise对象包装成一个新的Promise对象，当所有Promise对象都变为fulfilled时，新的Promise对象的状态才变为fulfilled，它的值是所有Promise对象的值组成的数组；当有一个Promise对象变为rejected时，新的Promise对象的状态就变为rejected，它的值是第一个被rejected的Promise对象的值。
Promise.all = function(promises) {
return new Promise(function(resolve, reject) {
var resolvedCounter = 0;
var promiseNum = promises.length;
var resolvedValues = new Array(promiseNum);
for (var i = 0; i < promiseNum; i++) {
(function(i) {
Promise.resolve(promises[i]).then(function(value) {
resolvedCounter++;
resolvedValues[i] = value;
if (resolvedCounter == promiseNum) {
resolve(resolvedValues);
}
}, function(reason) {
reject(reason);
});
})(i);
}
});
};
- Promise.race方法
Promise.race方法用于将多个Promise对象包装成一个新的Promise对象，当有一个Promise对象变为fulfilled或rejected时，新的Promise对象的状态就变为fulfilled或rejected，它的值是第一个变为fulfilled或rejected的Promise对象的值。
Promise.race = function(promises) {
return new Promise(function(resolve, reject) {
for (var i = 0; i < promises.length; i++) {
Promise.resolve(promises[i]).then(function(value) {
resolve(value);
}, function(reason) {
reject(reason);
});
}
});
};
