Calculate Mathematical Expression

eval_math_expr.js

let resolve = (tokens, vars) => {  
  let getVal = (t, vars) => {
    if (isIdent(t)) {
      if (t in vars) return +vars[t];
      throw Error(`variable ${t} is not defined`);
    }
    return +t;
  };  
  let isOp = (str) => op.s.indexOf(str) !== -1;
  let isIdent = (str) => !!str.match(/^[a-z_][a-z_\d]*$/i);
  let opSt = (fn) => {
    return (t, i, vars) => {
      let res = fn(getVal(t[i-1], vars), getVal(t[i+1], vars));
      t.splice(i - 1, 3, `${res}`)
      return -1;
    };
  };
  let = op = {
    s: '(*/%+-=',
    p: [0, 1, 1, 1, 2, 2, 3],
    fn: [
      (t, i, vars) => {
        let c = 0;
        let idx = t.findIndex((t, ii) => {
          if (ii < i) return false;
          c += ({ '(': 1, ')': -1 })[t] || 0;
          return c === 0;
        });
        let res = resolve(t.slice(i+1, idx), vars);
        t.splice(i, idx - i + 1, `${res}`);
        return 0;
      },
      opSt((a, b) => a * b),
      opSt((a, b) => a / b),
      opSt((a, b) => a % b),
      opSt((a, b) => a + b),
      opSt((a, b) => a - b),
      (t, i, vars) => {
        vars[t[i-1]] = t[i+1];
        tokens.splice(i - 1, 3, t[i+1]);
        return -1;
      }
    ]
  };
  
  let maxP = Math.max(...op.p);
  for (let p = 0; p <= maxP; ++p) {
    for (let i = 0; i < tokens.length; ++i) {
      if (isIdent(tokens[i])) {
        if (tokens[i] in vars) {
          tokens[i] = vars[tokens[i]];
        }
      }
      if (isOp(tokens[i])) {
        let idx = op.s.indexOf(tokens[i]);
        if (op.p[idx] === p) {
          i += op.fn[idx](tokens, i, vars);
        }
      }
    }
  }

  return getVal(tokens[0]);
};