Node.js Readline暂停代码

Question

Sorry I've just dipped my feet in JavaScript, and currently trying to collect user input from the console. I have a code that looks like this:

main = () => {
    var num = getInput();
    console.log(num);
}

getInput = () => {
    const readline = require('readline');
    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout
    });
    rl.question('Lemme test: ', (ans) => {
        rl.close();
        return ans;
    });
}

main();

So if I try to run this, it'll show:

Lemme test: undefined

and then wait for me to input. Apparently console.log(num); ran before getInput(); was finished, or getInput(); spits out an undefined then ask for input.

BTW switching rl.close(); and return ans; does not work.

Why does this happen?

Answer 1

This happened because node doesn't wait for readline to get input - like almost everything in Node, it is asynchronous. So it fires off the request for user input and continues on with the program. Once there is some user input, if fires the callback you gave it. So to deal with this you need to manage the use input in the callback or have a function that is called in the callback to deal with the input asynchronously. For example:

main = () => {
    var num = getInput();
}

dealWithInput = (str) => {
    console.log(str)
}

getInput = () => {
    const readline = require('readline');
    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout
    });
    rl.question('Lemme test: ', (ans) => {
        rl.close();
        dealWithInput(ans);
    });
}

main();

Answer 2

That's because the getInput function is a callback which means that it's executed at some point in the future so the console.log , which is not a callback, gets executed before.

A function or a method it's a callback when you assign it to another function like this:

let example = () => {
   // This is a callback
}

doSomething(result => {
   // This is a callback too because its the same as doing the following
})

doSomething(function(result){
   // This is a callback without arrow functions
})