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面试中碰见手写Promise就抓瞎，研究了一下根据Promise A+规范手写了个Promise

//新增三种状态 const STATUS_PENDING = "pending"; const STATUS_FULFILLED = "fulfilled"; const STATUS_REJECTED = "rejected"; class _Promise { constructor(executor = () => {}) { //立即执行构造函数且状态变为pending this.status = STATUS_PENDING; this.value = undefined; this.reason = undefined; const resolve = (value) => { //只有当状态是pending时执行resolve后状态变为fullfilled， if(this.status === STATUS_PENDING){ this.status = STATUS_FULFILLED this.value = value } }; const reject = (reason) => { //只有当状态是pending时执行reject后，状态变为rejected if(this.status === STATUS_PENDING){ this.status = STATUS_REJECTED this.reason = reason } }; executor(resolve, reject); } then(onFulfilled, onRejected){ if(onFulfilled && typeof onFulfilled === 'function'){ onFulfilled(this.value) } if(onRejected && typeof onRejected === 'function'){ onRejected(this.reason) } } catch(onRejected) { if(onRejected && typeof onRejected === 'function'){ onRejected(this.reason) } } }