promise 手写

以下是手写的Promise实现:

class MyPromise {
    constructor(executor) {
        this.status = 'pending';
        this.value = undefined;
        this.reason = undefined;
        this.onResolvedCallbacks = [];
        this.onRejectedCallbacks = [];

        const resolve = (value) => {
            if (this.status === 'pending') {
                this.status = 'fulfilled';
                this.value = value;
                this.onResolvedCallbacks.forEach(fn => fn());
            }
        };

        const reject = (reason) => {
            if (this.status === 'pending') {
                this.status = 'rejected';
                this.reason = reason;
                this.onRejectedCallbacks.forEach(fn => fn());
            }
        };

        try {
            executor(resolve, reject);
        } catch (error) {
            reject(error);
        }
    }

    then(onFulfilled, onRejected) {
        onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value;
        onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason };

        const promise2 = new MyPromise((resolve, reject) => {
            if (this.status === 'fulfilled') {
                setTimeout(() => {
                    try {
                        const x = onFulfilled(this.value);
                        resolvePromise(promise2, x, resolve, reject);
                    } catch (error) {
                        reject(error);
                    }
                }, 0);
            }

            if (this.status === 'rejected') {
                setTimeout(() => {
                    try {
                        const x = onRejected(this.reason);
                        resolvePromise(promise2, x, resolve, reject);
                    } catch (error) {
                        reject(error);
                    }
                }, 0);
            }

            if (this.status === 'pending') {
                this.onResolvedCallbacks.push(() => {
                    setTimeout(() => {
                        try {
                            const x = onFulfilled(this.value);
                            resolvePromise(promise2, x, resolve, reject);
                        } catch (error) {
                            reject(error);
                        }
                    }, 0);
                });

                this.onRejectedCallbacks.push(() => {
                    setTimeout(() => {
                        try {
                            const x = onRejected(this.reason);
                            resolvePromise(promise2, x, resolve, reject);
                        } catch (error) {
                            reject(error);
                        }
                    }, 0);
                });
            }
        });

        return promise2;
    }

    catch(onRejected) {
        return this.then(null, onRejected);
    }
}

function resolvePromise(promise2, x, resolve, reject) {
    if (promise2 === x) {
        return reject(new TypeError('Circular reference'));
    }

    if (x instanceof MyPromise) {
        x.then(value => {
            resolvePromise(promise2, value, resolve, reject);
        }, reject);
    } else if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
        let then;
        try {
            then = x.then;
            if (typeof then === 'function') {
                then.call(x, value => {
                    resolvePromise(promise2, value, resolve, reject);
                }, reason => {
                    reject(reason);
                });
            } else {
                resolve(x);
            }
        } catch (error) {
            reject(error);
        }
    } else {
        resolve(x);
    }
}

以上是一个基本的Promise实现，它具有Promise的基本特性，包括状态管理、异步操作、链式调用、错误处理等。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
- *1* [超详细！一次性搞懂手写Promise全过程](https://blog.csdn.net/qq_41852789/article/details/128453341)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v93^chatsearchT3_2"}}] [.reference_item style="max-width: 50%"]
- *2* *3* [Promise学习-手写一个promise](https://blog.csdn.net/qq_42161935/article/details/120672260)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v93^chatsearchT3_2"}}] [.reference_item style="max-width: 50%"]
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