JavaScript的使用功能，而循环条件

Z时代
2024-01-10
分类：问答

全球

定义的变量：JavaScript的使用功能，而循环条件

var domArray = [] //will have multiple string 
var onQueueDom = [] //will have 1 string only 
var onQueueDomStatus = ["N"] //will one of the status: "N", "P","D" 
var processedNum = 0

我创建这将返回真或假，它会等待3秒只运行，如果其他功能：

function checkIfPending(){ 
    console.log('checkIfPending being executed ') 
    setTimeout(function(){ 
     if(onQueueDomStatus[0] == "D"){ 
     console.log("Process Done, returning True") 
     return true 
     console.log("True has been returned, you shouldn't seeing this") 
     } 
     else if(onQueueDomStatus[0] !== "D" || onQueueDomStatus[0] !== "N" && onQueueDom !== ""){ 
      console.log("Still Processing, will re-check in 3 second") 
      return false 
     } 
     else { 
      console.log("No domain on Queue but status not clear") 
      console.log("Clearing status...") 
      onQueueDomStatus[0] = "D" 
      console.log('setting onQueueDomStatus to ' + onQueueDomStatus[0]) 
      console.log("Status changed to D & returning True") 
      return true 
     } 
    }, 3000); 
}

我想使用上述函数作为while循环的条件，但它不会处理While循环中的代码，即使onQueueDomStatus [0] ==“D”：

while(checkIfPending() == true){ 
       console.log('while loop is running') 
       onQueueDomStatus[0] = "N" 
       console.log('setting onQueueDomStatus to ' + onQueueDomStatus[0]) 
       movetoQueue() 
       console.log('Executing movetoQueue') 
      }

假设onQueueDomStatus [0]始终是“D”，但它仍然不起作用。

问题：while循环会每次等待3秒钟执行吗？

回答：

checkIfPending没有返回值，所以调用它会导致值undefined。你传递的回调setTimeout有一个返回值（虽然setTimeout忽略它），但checkIfPending没有。 checkIfPending不能返回它来自异步操作（more here）的任何值。

while是同步控制流程结构*不能将异步函数的结果用作while中的条件。

编辑：gurvinder372有done a good job向您展示如何重组事物来处理异步性。

*的while的语义可以异步做出了ES2017 + async功能里面如果您使用await，但在幕后，真正的情况是，该功能被重写不使用while。

回答：

您需要调用，而不是返回true或false

function checkIfPending(exitCallback, trueCallback){ 
    console.log('checkIfPending being executed ') 
    setTimeout(function(){ 
     if(onQueueDomStatus[0] == "D"){ 
      trueCallback(); //invoke callback which signals that checkPending should continue 
      checkIfPending(exitCallback, trueCallback); 
     } 
     else if(onQueueDomStatus[0] !== "D" || onQueueDomStatus[0] !== "N" && onQueueDom !== ""){ 
      exitCallback(); 
     } 
     else 
     { 
      onQueueDomStatus[0] = "D"; 
      trueCallback(); //invoke callback which signals that checkPending should continue 
      checkIfPending(exitCallback, trueCallback); 
     } 
    }, 3000); 
}

回调，并以此作为

checkIfPending(function(){ 
    //console.log("while loop ended"); 
}, function(){ 
    onQueueDomStatus[0] = "N"; 
    movetoQueue(); 
})

回答：

此代码将帮助您

function getPromise(){  
\t return new Promise((resolve,reject)=>{  
\t setTimeout(function(){  
     if(onQueueDomStatus[0] == "D"){  
     console.log("Process Done, returning True")  
     resolve(true)  
     }  
     else if(onQueueDomStatus[0] !== "D" || onQueueDomStatus[0] !== "N" && onQueueDom !== ""){  
      console.log("Still Processing, will re-check in 3 second")  
      resolve(false)  
     }  
     else {  
      console.log("No domain on Queue but status not clear")  
      console.log("Clearing status...")  
      onQueueDomStatus[0] = "D"  
      console.log('setting onQueueDomStatus to ' + onQueueDomStatus[0])  
      console.log("Status changed to D & returning True")  
      resolve(true)  
     }  
    }, 3000);  
})  
}  
async function final(){  
\t var d = await getPromise(); // d will contain boolean after specified delay  
\t while(d == true){  
\t \t onQueueDomStatus[0] = "N"  
\t \t movetoQueue();  
\t \t d = await getPromise(); \t  
\t }  }