关于javascript:手写Promise

Promise的用法：

let p = new Promise(function(resolve, reject){console.log(111)
    setTimeout(function(){resolve(2)
    }, 1000)
})
p.then(function(res){console.log('suc',res)
},function(err){console.log('err',err)
})

能够看到 new 了一个 Promise, 外面有一个回调函数，回调函数里有 2 个参数，别离又是另外 2 个函数。其实外面很多也都是回调函数的封装调用。

const [PENDING,RESOLVED,REJECTED]=["PENDING","RESOLVED","REJECTED"];`

class Promise1 {constructor(executor){
        state = PENDING;
        value = undefined;
        reason = undefined;
        
        resolve = (value) => {if(this.state === PENDING){
                this.state = RESOLVED;
                this.value = value;
            }
        }
        reject = (reason) => {if(this.state === PENDING){
                this.state = REJECTED;
                this.value = reason;
            }
        }
        
        try{executor(this.resolve,this.reject)
        }catch(e){reject(e)
        }
    }
 }

测试一下执行状况：

let p1 = new Promise1((resolve,reject)=>{//function:executor
    console.log(111)
    setTimeout(()=>{resolve(111)//this.resolve, 间接传参调用
    }, 1000)
})

上面就是 then 了

class Promise1 {constructor(executor){
        ...
        onResolvedCallbacks=[]; //then,pending 里先存起来
        onRejectedCallbacks=[];

        ...

        resolve = (value) => {if(this.state === PENDING){
                ...
                //executor 里如果是异步会把 then 里存起来, 而后 executor 走完了 resolved 的话, 会再调 then 里回调函数
                this.onResolvedCallbacks.forEach(fn => fn(value))
            }
        }
        reject = (reason) => {if(this.state === PENDING){
                ...
               this.onRejectedCallbacks.forEach(fn => fn(reason))
            }
        }
    }
    
    then(onResolved,onRejected){
        onResolved = typeof onResolved === 'function' ? onResolved : value => value;
        onRejected = typeof onRejected === 'function' ? onRejected : reason => {throw reason};
        
        switch(this.state){
            case RESOLVED : // 状态变了, 就间接把后果给进来
                onResolved(this.value);
            case REJECTED :
                onRejected(this.reason);
            case PENDING : 
            // 如果是 pending 的话, 阐明 executor 还没走完, 先存起来, 走完再调用
                this.onResolvedCallbacks.push(onResolved);
                this.onRejectedCallbacks.push(onRejected)
        }
    }
}

测试一下：

let p2 = new Promise1((resolve, reject) => {setTimeout(()=>{resolve(222)
    }, 1000)
})
p2.then(res => console.log(res, 'res')) //222

链式调用：

class Promise1 {
    ...
    
    
    then(onResolved,onRejected){
        ...
        // 返回一个 promise1
        let newPromise=new Promise1((resolve,reject)=>{switch(this.state){
                case RESOLVED : // 状态变了, 就间接把后果给进来
                    setTimeout(()=>{//then, 放前面解决
                        try{let reValue=onResolved(this.value);
 rePromise(newPromise,reValue,resolve,reject)
                        }catch(e){reject(e)}
                    },0)
                    
                case REJECTED :
                    setTimeout(()=>{
                        try{let reValue=onRejected(this.reason);
                            rePromise(newPromise,reValue,resolve,reject)
                        }catch(e){reject(e)}
                    },0)
                    
                case PENDING : 
                // 如果是 pending 的话, 阐明 executor 还没走完, 先存起来, 走完再调用
                    this.onResolvedCallbacks.push(()=>{setTimeout(()=>{
                            try{let reValue=onResolved(this.value);
 rePromise(newPromise,reValue,resolve,reject)
                            }catch(e){reject(e)}
                        },0)
                    });
                    this.onRejectedCallbacks.push(()=>{setTimeout(()=>{
                            try{let reValue=onRejected(this.reason);
                            rePromise(newPromise,reValue,resolve,reject)
                            }catch(e){reject(e)}
                        },0)
                    })
            }
        })
        return newPromise;
    }
}

//newPromise: 新的 promise1 对象,reValue: 上一个 then 的返回值,newPromise.resolve,newPromise.reject
function rePromise(newPromise,reValue,resolve,reject){// 相当于把本人 return 进来了,let p2 = p.then(res => p2);
    if(newPromise === reValue){reject(new TypeError('循环了'))
    }
    if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
        try{
    // 判断一下 reValue 是不是 promise1, 是个对象, 还带有 then 函数
    // 如果有 then 并是个函数, 就调 then; 
            let then = reValue.then;
            if(typeof then === 'function'){
                then.call(reValue, res=>{resolve(res)
                },err=>{reject(err)
                })
            }else{//then 不是函数, 间接用 resolve 返回值
                resolve(reValue)
            }
        }catch(e){reject(e)
        }
    }else{// 只是个一般值
        resolve(reValue)
    }
}

测试一下：

p3.then(res => {console.log(res)
    return new Promise1((resolve, reject) => {resolve('p3')
    })
}).then(res => console.log(res, 'pp3'))//p3

但还有个问题

...

function rePromise(newPromise,reValue,resolve,reject){
    ...
    let called;// 是否调用过, 避免屡次调用
    if(reValue !== null && (typeof reValue === 'object' || typeof reValue === 'function')){
        try{
   
            let then = reValue.then;
            if(typeof then === 'function'){
                then.call(reValue, res=>{if(called) return;
                    called = true;
                   //promise1 就持续下一个, 直到 then 走完, 变成一般的值 
                   rePromise(newPromise,res,resolve,reject)
                },err=>{if(called) return;
                    called = true;
                    reject(err)
                })
            }else{//then 不是函数, 间接用 resolve 返回值
                resolve(reValue)
            }
        }catch(e){if(called) return;
            called = true;
            reject(e)
        }
    }else{// 只是个一般值
        resolve(reValue)
    }
}

测试一下

    let p1=new Promise1((resolve,reject)=>{resolve(111)
    })
    let p2=p1.then(res=>{console.log(res,'p1')},err=>console.log(err,'p1'));
    p2.then(res=>console.log(res,'p2'))
    .then(res=>console.log(res,'p3))

还有其它属性：

class Promise1 {
    ...
    catch(onReject){return this.then(null,onReject)
    }
    defer(){let defer={};
        defer.promise=new Promise1((resolve,reject)=>{
            defer.resolve=resolve;
            defer.reject=reject;
        })
        return defer;
    }
    deffered(){return this.defer;}
    all(promises){return new Promise1((resolve,reject)=>{let done=gen(promises.length,resolve);
            for(let i=0;i<promises.length,i++){promises[i].then(res=>{done(i,res)
                },reject)
            }
        })
    }
    race(promises){return new Promise1((resolve,reject)=>{for(let i=0;i<promises.length;i++){promises[i].then(resolve,reject)
            }
        })
    }
    resolve(value){return new Promise1((resolve,reject)=>{resolve(value)
        })
    }
    reject(reason){return new Promise1((resolve,reject)=>{reject(reason)
        })
    }
}

function gen(times,cb){const result=[];
    let count=0;
    return (i,data)=>{result[i]=data;
        count+=1;
        if(count===times){cb(result)
        }
    }
}