javascript - 为什么返回 'Undefined?'

Question

我正在学习一些 Node 核心模块，并且我编写了一个小命令行工具来测试 readline模块但在我的console.log()上输出，我也收到 undefined在它下面:/

这是我的代码..

var rl = require('readline');

var prompts = rl.createInterface(process.stdin, process.stdout);

prompts.question("What is your favourite Star Wars movie? ", function (movie) {

    var message = '';

    if (movie = 1) {
        message = console.log("Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!");
    } else if (movie > 3) {
        message = console.log("They were great movies!");
    } else {
        message = console.log("Get out...");
    }

  console.log(message);

  prompts.close();
});

这是我在控制台中看到的内容..

What is your favourite Star Wars movie? 1
Really!!?!?? Episode1 ??!?!!?!?!, Jar Jar Binks was a total dick!
undefined

我为什么要回来undefined ？

Answer 1

Why am I getting back undefined?

因为 console.log 没有返回值，所以您将 undefined 分配给 message。

由于您稍后要输出消息，因此只需从您设置消息的行中删除console.log 调用即可。例如，改变

message = console.log("Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!");

至

message = "Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!";

<小时/>

旁注:您的台词

if (movie = 1) {

将数字 1 分配给movie，然后测试结果 (1) 以查看其是否真实。因此，无论您输入什么，它总是会选择该分支。您的意思可能是:

if (movie == 1) {

...尽管我建议不要依赖用户提供的输入的隐式类型强制，因此我会将其放在该回调的顶部附近:

movie = parseInt(movie, 10);