promise实现原理

若没有链式then,then函数就不用return new Promise递归

function Promise(fn) {
    var state = "pending"
    var value = null,
        callbacks = [];  //callbacks为数组，因为可能同时有很多个回调

    this.then = function (onFulfilled, onRejected) {
        return new Promise(function(resolve, reject){
            handle({
                onFulfilled: onFulfilled || null,
                resolve: resolve,
                onRejected: onRejected || null,
                reject: reject,
            })
        })
    };
    function handle(callback){
        if(state === "pending"){
            callbacks.push(callback);
            return;
        }
        let cb = state === 'fulfilled'? callback.onFulfilled: callback.onRejected,
        ret;
        if(cb === null){
            cb = state == 'fulfilled'? callback.resolve: callback.reject;
            cb(value)
            return;
        }
        try{
            ret = cb(value);
            callback.resolve(ret);
        }catch(e){
            callback.reject(e)
        }

    }
    function resolve(newValue) {
        if(newValue && (typeof newValue === 'object' || typeof newValue === 'function')){
            let then = newValue.then
            if(typeof then === 'function'){
                then.call(newValue, resolve, reject);
                return;
            }
        }
        value = newValue
        state = "fulfilled"
        execute()    
    }
    function reject(reason){
        state = 'rejected'
        value = reason
        execute()
    }
    //用于操作resolve和reject
    function execute(){
        setTimeout(function(){
            callbacks.forEach(function (callback) {
                handle(callback)
            });
        },0)  
    }
    fn(resolve, reject);
}

function getUserId() {
    return new Promise(function (resolve) {
        resolve("success")
        console.log('success')
        // reject("reason")
    })
}
function getUserJobById(id){
    return new Promise(function (resolve){
        resolve("UserJob")
    })
}
getUserId().then(getUserJobById).then(function(id) {
    console.log(id)
})
function testError(){
    return new Promise(function(resolve, reject){
        reject("reason")
    })
}
testError().then(function(){

},function(error){
    console.log("error:" + error)
})