lookupObjectValue.js

const lookUpObjectValue = (obj, str) => {

    const validProp = c => (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c === '_';
    const isNum = c => c >= '0' && c <= '9';
    const errored = i => {
        const cursor = `${str}\n${' '.repeat(i)}^`;
        console.error(`Invalid accessor:\n${cursor}`);
    };

    const length = str.length;
    for (let i = 0; i < length; i++) {
        const ch = str[i];

        if (validProp(ch)) {
            let consumed = ch;
            for (; validProp(str[i + 1]) || isNum(str[i + 1]);
                   consumed += str[++i]) ;

            obj = obj[consumed];
        } else if (ch === '[') {
            const next = str[++i];

            // Resolve string or index
            if (next === '\'' || next === '"') {
                i++;
                let consumed = '';
                for (; str[i] !== next || str[i - 1] === '\\';
                       consumed += str[i++]) ;

                obj = obj[consumed];
                i++;
            } else {
                let consumed = next;
                for (; isNum(str[i + 1]);
                       consumed += str[++i]) ;

                const parsed = Number(consumed || NaN);
                if (str[++i] === ']' && !isNaN(parsed)) {
                    obj = obj[parsed];
                } else {
                    return errored(i);
                }
            }
        } else if (isNum(ch)) {
            return errored(i);
        } else {
            continue;
        }

        if (i !== length - 1 && str[i + 1] !== '.' && str[i + 1] !== '[') {
            return errored(i);
        }
    }

    return obj;
};