我的首个项目：石头剪刀布JavaScript代码无法运行，请求排查

错误1：错误使用parseInt处理用户输入 #

你用parseInt(prompt("input rock paper or scissors"))处理用户输入，但用户输入的是字符串（比如"rock"），parseInt会把非数字字符串转换成NaN，导致playerSelection的值是NaN，和computerSelection的字符串永远无法匹配，所有条件都不触发。

修复： 去掉parseInt，直接获取用户输入，同时统一转成小写，避免大小写不匹配的问题：

let playerSelection = prompt("input rock paper or scissors").toLowerCase();

错误2：平局分支无返回值 #

在playRound函数的平局条件里，你只执行了console.log("its a tie")，但没有返回值，导致平局时console.log(playRound(...))会输出undefined。

修复： 将平局的console.log改为return：

if (playerSelection == computerSelection) {
    return "its a tie";
}

完整修复后的代码 #

function playRound(playerSelection, computerSelection) {
    if (playerSelection == computerSelection) {
        return "its a tie";
    } else if (playerSelection == "rock" && computerSelection == "paper") {
        return "you lose";
    } else if (playerSelection == "rock" && computerSelection == "scissors") {
        return "you win";
    } else if (playerSelection == "paper" && computerSelection == "scissors") {
        return "you lose";
    } else if (playerSelection == "paper" && computerSelection == "rock") {
        return "you win";
    } else if (playerSelection == "scissors" && computerSelection == "rock") {
        return "you lose";
    } else if (playerSelection == "scissors" && computerSelection == "paper") {
        return "you win";
    }
}

let playerSelection = prompt("input rock paper or scissors").toLowerCase();
let computerSelection = Math.random();

if (computerSelection < 0.34) {
    computerSelection = "rock";
} else if (computerSelection <= 0.67) {
    computerSelection = "paper";
} else {
    computerSelection = "scissors";
}

console.log(playRound(playerSelection, computerSelection));

内容的提问来源于stack exchange，提问作者eret 007