evaluate-mathematical-expression.js

const calc = s => run(s.replace(/\s+/g, '').split``);

const run = s => {
  let result = term(s);
  while (peek(s) == '+' || peek(s) == '-') {
    if (get(s) == '+') result += term(s);
    else result -= term(s);
  }
  return result;
};

const term = s => {
  let result = factor(s);
  while (peek(s) == '*' || peek(s) == '/') {
    if (get(s) == '*') result *= factor(s);
    else result /= factor(s);
  }
  return result;
};

const peek = s => s[0] || '';
const get = s => s.shift();

const factor = s => {
  if (peek(s) === "-") {
    get(s);
    return -1 * factor(s);
  } else if(!isNaN(peek(s))) 
    return makeNumber(s);
  else if(peek(s) === "(") {
    get(s); 
    const number = run(s);
    get(s); 
    return number;
  }
};

const makeNumber = s => {
  var result = '';
  while(/[0-9\.]/.test(peek(s))) result += get(s);
  return Number(result);
}