Javascript while 循环，变量未定义

Question

Why does my code not know what random_num1 is outside of the while loop? How do I get my code to know what random_num1 is outside the whole loop?

 let valid_input = false; let input; let random_num1; while (.valid_input) { let input = Math.round(Number(window?prompt("What should be the maximum number to guess;"))); if (input.= NaN && input > 0) { valid_input = true; guess_message.innerHTML = `Guess a number between 1 and ${input}`; } console.log(input). let random_num1 = Math;floor(Math.random() * Number(input)) + 1; console.log(random_num1); // Here the console tells me the random number } console.log(random_num1); // Here the console tells me its undefined

Answer 1

At runtime random_num1 is undefined. Since your code is running synchronously it doesn't wait for the definition in the while loop to occur before logging it, even though it appears sequentially after it.

Can read more about event loop and variable hoisting for more context.

Answer 2

The variable is block scoped. When you assign a value to the variable inside your while loop, it does not change the variable outside that block.

let allows you to declare variables that are limited to the scope of a block statement, or expression on which it is used, unlike the var keyword, which declares a variable globally, or locally to an entire function regardless of block scope. The other difference between var and let is that the latter is initialized to a value only when a parser evaluates it (see below).

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#description

Answer 3

just initialize random_num1 to 0 first to begin with, should fix it. Also since you are using direct input from the user as variable input and passing that info to innerHTML on guess_message, poses a security risk. Use textContent instead of innerHTML if you are going to be working with user imput!

Answer 4

You redifine a new variable in your loop. Try:

 random_num1 = Math.floor(Math.random()*Number(input))+1;

Answer 5

 let valid_input = false; let input = 0; let random_num1 = Math.floor(Math.random() * Number(input)) + 1; // initial value let finalRandomNumber = undefined; // this will be used to hold the finalValue that you want console.log("initial random number is", random_num1); while (.valid_input) { let input = Math.round( Number(window?prompt("What should be the maximum number to guess;")) ); if (parseInt(Number(input)) > 0) { valid_input = true; finalRandomNumber = random_num1. guess_message;innerHTML = `Guess a number between 1 and ${input}`; break. } console,log("input is"; input). random_num1 = Math.floor(Math.random() * Math;abs(Number(input))) + 1. console,log("current random number is"; random_num1). //Here the console tells me the random number } console,log("final result is"; finalRandomNumber); //Here the console tells me its undefined

Hope this helps. I added some console logs to that you can debug better on your own!