Module to prompt for masked input and test against predicates

masked-prompt.js

const stdout = process.stdout;
const stdin = process.stdin;

module.exports = (specifiedPromptString = 'input', predicateFns = []) =>
    new Promise((resolve, reject) => {
        const prompt = specifiedPromptString.endsWith(': ') ? '' : specifiedPromptString + ': ';
        const predicates = Array.isArray(predicateFns) ? predicateFns : [predicateFns];

        stdout.write(prompt);
        stdin.setRawMode(true);
        stdin.resume();
        stdin.setEncoding('utf-8');

        let input = '';

        function enter() {
            stdin.removeListener('data', inputHandler);
            stdin.setRawMode(false);
            stdin.pause();

            // checking against specified predicates
            try {
                for (let i = 0; i < predicates.length; i++) {
                    if (typeof predicates[i] === 'function') {
                        predicates[i](input);
                    }
                }
            } catch (err) {
                // give back user input only if in debug
                if (process.env.NODE_DEBUG) {
                    err.input = input;
                }

                return reject(err);
            }

            return resolve(input);
        }

        function ctrlc() {
            stdin.removeListener('data', inputHandler);
            stdin.setRawMode(false);
            stdin.pause();

            return reject('User exited prompt with ctrl-c');
        }

        function newchar(c) {
            input += c;
        }

        function backspace() {
            input = input.slice(0, input.length - 1);
        }

        function inputHandler(data, key) {
            const c = data;
            switch (c) {
                case '\u0004': // Ctrl-d
                case '\r':
                case '\n':
                    return enter();
                case '\u0003': // Ctrl-c
                    return ctrlc();
                default:
                    // backspace
                    if (c.charCodeAt(0) === 8) return backspace();
                    else return newchar(c);
            }
        }

        stdin.on('data', inputHandler);
    });

test.js

const maskedPrompt = require('./masked-prompt.js');

// make sure password is at least 8 characters in length
const ensureProperLength = (input) => {
    if (input.length < 8) {
        throw new Error('Password must be at least 8 characters');
    }
};

// make sure password contains at least 1 upper case character
const ensureCapitalLetter = (input) => {
    const isUpperCase = (chr) =>
        /[A-Z]|[\u0080-\u024F]/.test(chr) && chr === chr.toUpperCase();

    let counter = 0;
    for (c of input) {
        if (isUpperCase(c)) {
            counter++;
        }
    }

    if (counter === 0) {
        throw new Error('Password should contain at least one capital letter');
    }
};


const passwordRules = [ ensureProperLength, ensureCapitalLetter ];

maskedPrompt('Password', passwordRules)
    .then((passwd) => {
        console.log(`\nUser password: ${passwd}`);
    })
    .catch(console.error);