AFD in Node.js

'use strict'

/**
 * @param {Number} pos
 * @param {String} letter
 * @return {Number}
 */
const switchState = (pos, letter) => {
    const states = (currentPos) => ({
        1: letter === 'a' ? 2 : 3,
        2: letter === 'b' ? 4 : 3,
        3: 3,
        4: 4,
    }[currentPos]);

    return states(pos);
}

/**
 * @param {String} words
 * @returns {Boolean}
 */
const validateDfa = (words) => {
    let pos = 1;
    const letters = words.split('');

    for (const letter of letters) {
        pos = switchState(pos, letter);
    }

    return pos === 4;
}