I'm simply trying to wait for a user to enter a password and then use it before moving on the rest of my code. The error is Cannot read property 'then' of undefined.

let rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

rl.question('Password: ', password => {
    rl.close();
    return decrypt(password);
}).then(data =>{
    console.log(data);
});

function decrypt( password ) {
    return new Promise((resolve) => {
        //do stuff
        resolve(data);
    });
}

I'm simply trying to wait for a user to enter a password and then use it before moving on the rest of my code. The error is Cannot read property 'then' of undefined.

let rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

rl.question('Password: ', password => {
    rl.close();
    return decrypt(password);
}).then(data =>{
    console.log(data);
});

function decrypt( password ) {
    return new Promise((resolve) => {
        //do stuff
        resolve(data);
    });
}

• javascript
• node.js
• promise
• readline

• 1 It looks like rl.question does not return promise in vanilla Node.Js (I'm assuming the question is about node.js). Actually, it returns nothing, see source code. So I'd expect that there is no then method on it. So if you really need a promise, I think you'll have to implement it manually. – yeputons Commented Feb 7, 2017 at 2:27

2 Answers 2

Readline's question() function does not return Promise or result of the arrow function. So, you cannot use then() with it. You could simply do

rl.question('Password: ', (password) => {
    rl.close();
    decrypt(password).then(data => {
       console.log(data);
    });
});

If you really need to build a chain with Promise, you can pose your code differently:

new Promise((resolve) => {
    rl.question('Password: ', (password) => {
        rl.close();
        resolve(password);
    });
}).then((password) => {
   return decrypt(password); //returns Promise
}).then((data) => {
   console.log(data); 
});

You probably should not forget about the .catch(), otherwise either solution works, the choice should be based on which code will be easier to read.

You may want to look at a couple of additional promise usage patterns

And if you are scared of using callbacks and the possibility of callback hell. You can check readline-sync package