Lexer

    const makeToken = function (type, value) {
      return {
        type,
        value
      };
    }

    const EOF = makeToken("(end)", 0);

    const operators = ["+", "-", "/", "*", "^", "(", ")", "="].reduce((aggr, next, index) => {
      aggr[next] = makeToken("(operator)", next);
      return aggr;
    }, {});


    const lexer = module.exports = function (str) {
      const tokens = [];

      let from = 0, to = 0, char, type;

      while (from < str.length) {
        char = str.charAt(from);

        if (char === "\u0020" || char === "\r" ||
          char === "\n" || char === "\t") {
          from++;
          continue;
        }

        /* check if number*/
        if (char >= "0" && char <= "9") {
          type = "(number)";
          to = from + 1;
          let numStr = char;
          let next = str.charAt(to);

          while (to < str.length && next >= "0" && next <= "9") {
            numStr += next;
            to++;
            next = str.charAt(to);
          }

          if (next === ".") {
            numStr += next;
            to += 1;
            next = str.charAt(to);

            while (to < str.length && next >= "0" && next <= "9") {
              numStr += next;
              to++;
              next = str.charAt(to);
            }
          }

          if (next === "E" || next === "e") {
            numStr += next;
            to += 1;
            next = str.charAt(to);

            if (next === "+" || next === "-") {
              numStr += next;
              to += 1;
              next = str.charAt(to);
            }
            while (to < str.length && next >= "0" && next <= "9") {
              numStr += next;
              to++;
              next = str.charAt(to);
            }
          }

          tokens.push(makeToken(type, parseFloat(numStr)));
          from = to;
          continue;
        }

        //check if identifier
        if ((char >= "A" && char <= "Z") || (char >= "a" && char <= "z")) {
          type = "(id)";
          to = from + 1;
          char = str.charAt(to);

          while ((char >= "A" && char <= "Z") || (char >= "a" && char <= "z")) {
            char = str.charAt(to);
            to += 1;
          }

          tokens.push(makeToken(type, str.slice(from, to)));
          from = to;
          continue;
        }

        //check if operator
        if (operators[char]) {
          tokens.push(operators[char]);
        }
        else {
          tokens.push(makeToken("(unknown)", char));
        }

        from += 1;
      }

      tokens.push(EOF);

      return tokens;
    }