手寫系列 帶你實現乙個簡單的Promise

學習之前 需要先對promise有個基本了解哦,這裡都預設大家都是比較熟悉promise的

promise的基本骨架

promise的then

promise.then的多次呼叫

then鏈式呼叫

catch的實現

finally的實現

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
class zxpromise 
}const rejected = (reason) => 
}executor(resolve, rejected)
}}// 初步搭建好promise的construtor結構
const promise = new zxpromise((resolve, rejected) => )

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
class zxpromise )
}}		const rejected = (reason) => )
}}		executor(resolve, rejected)
}	then(onfufilled, onrejected) 
}// 接下來開始寫then方法
const promise = new zxpromise((resolve, rejected) => )
promise.then((res) => , (err) => )

p1.then((res) => );

p1.then((res) => );
settimeout(() => )
}, 1000);

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
class zxpromise );})}
}		const rejected = (reason) => )})}
}		executor(resolve, rejected)
}	// 接下來為了promise能夠多次呼叫 進行優化
then(onfufilled, onrejected) 
if (this.status === promise_status_rejected) 
if (this.status === promise_status_pending) 
}}

要想實現鏈式呼叫,那麼then方法肯定是將promise物件又給返回出來了,說到這了大家有沒有思路呢?

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
class zxpromise );})}
}		const rejected = (reason) => )})}
}		trycatch(err)
}then(onfufilled, onrejected)  catch (err) 
}if (this.status === promise_status_rejected)  catch (err) 
}if (this.status === promise_status_pending) );
} catch (err) 
try );
} catch (err) 
}})	}}
const promise = new zxpromise((resolve, rejected) => )
promise.then((res) => , (err) => ).then((res) => , (err) => )

catch方法實際上是then第二個引數的語法糖,說到這裡大家有沒有明白什麼呢?

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
const execfnwithcatcherror = (execfn, value, resolve, reject) =>  catch (err) 
}class zxpromise );})}
}const rejected = (reason) => )
return this.reason;})}
}executor(resolve, rejected)
}then(onfufilled, onrejected) 
onrejected = onrejected || defaultonrejected
return new zxpromise((resolve, rejected) => 
if (this.status === promise_status_rejected && onrejected) 
if (this.status === promise_status_pending) );
if (onrejected) );}}
})}
catch(onrejected) 
}

finally就是要在最後執行的函式,無論什麼情況,實現起來也是非常簡單

finally(fn) , () => );

}

const promise_status_pending = "promise_status_pending";

const promise_status_fulfilled = "promise_status_fulfilled";
const promise_status_rejected = "promise_status_rejected";
const execfnwithcatcherror = (execfn, value, resolve, reject) =>  catch (err) 
}class zxpromise );})}
}const rejected = (reason) => )
return this.reason;})}
}executor(resolve, rejected)
}then(onfufilled, onrejected) 
onrejected = onrejected || defaultonrejected
return new zxpromise((resolve, rejected) => 
if (this.status === promise_status_rejected && onrejected) 
if (this.status === promise_status_pending) );
if (onrejected) );}}
})}
catch(onrejected) 
finally(fn) , () => );
}}

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