20.js

/**
 * @param {string} s
 * @return {boolean}
 */
var isValid = function (s) {
    const stack = [];
    const pairs = {
        '{': '}',
        '(': ')',
        '[': ']'
    };

    for (let char of s) {
        if (pairs[char]) {
            stack.push(char);
        } else {
            const last = stack.pop();
            if (pairs[last] !== char) {
                return false;
            }
        }
    }

    return stack.length === 0;
};