recursive-descent.js

/*
    S -> aAe
    A -> bAd | c
*/
const parse = (() => {
    let tokens;
    let pos = 0;

    const peek = () => tokens[pos];
    const eat = token => {
        if (peek() !== token)
            throw `Invalid token ${peek()}. Expected ${token}`;
        pos++;
    };

    function S() {
        eat('a');
        A();
        eat('e');
    }

    function A() {
        if (peek() === 'b') {
            eat('b');
            A();
            eat('d');
        } else {
            eat('c');
        }
    }

    return input => {
        tokens = input;
        pos = 0;
        
        try {
            S();
            return true;
        } catch (err) {
            console.log(err);
            return false;
        }
    };
})();

console.log(parse('abcde'));
console.log(parse('ace'));
console.log(parse('abbbbcdddde'));
console.log(parse('abbcdde'));
console.log(parse('abbcde'));
console.log(parse('abccde'));