如何继续检查javascript中的值是否正确？

Question

I have a javascript code that asks a user to input an entry, prints out a list of entries, and deletes. The problem is that when the user inputs delete and than enters an invalid number the code says to enter a correct index but if the user inputs still an incorrect index value based on my specifications the code stops checking whether it passes the test. I need the code to constantly check if the input is valid for index even after the user keeps putting a wrong index number. The code must not exit unless the correct index value is entered. Here is the code

let action = prompt("What would you like to do");
const todo = []
let count=0
let tracker=0
let char = 'x'
while (action !== 'quit' && action !== 'q'){
    if (action === 'new'){
        action = prompt("What would you like to add");
        todo.push(action)
        tracker=0;
    }
    else if (action === 'list'){
        console.log(char.repeat(10))
        for (let elements of todo){
            console.log(`${tracker}: ${elements}`)
            tracker++
        }
        console.log(char.repeat(10))
        action = prompt("What would you like to do");
    }
    else if (action === 'delete'){
        let index = parseInt(prompt("Enter the index of the todo you would like to delete"))
        if (index<todo.length && index >-1 && index!==null){
            todo.splice(index,1)
            tracker=0;
        }
        else{
                let index = parseInt(prompt("Enter a correct index"))
            }
        action = prompt("What would you like to do");
    }
    else
    {
        action = prompt("What would you like to do"); 
    }
}
console.log("Ok quit the app")

Answer 1

Replace this part:

let index = parseInt(prompt("Enter the index of the todo you would like to delete"))
if (index<todo.length && index >-1 && index!==null){
    todo.splice(index,1)
    tracker=0;
}
else{
        let index = parseInt(prompt("Enter a correct index"))
    }

With this -- using a loop:

let index = parseInt(prompt("Enter the index of the todo you would like to delete"));
while (!(index < todo.length && index >= 0)) {
    index = parseInt(prompt("Enter a correct index"));
}  
todo.splice(index, 1);
tracker = 0;

Note: parseInt will never return null , so no need to check for that. Instead, it could return NaN . But as any comparison with NaN will evaluate to false , the negation of that (using ! ) will evaluate to true .