Caculator modified from pegjs online example

caculator.pegjs

Expression
  = head:Term tail:(_ ("+" / "-") _ Term)* {
      var result = head, i;

      for (i = 0; i < tail.length; i++) {
        if (tail[i][1] === "+") { result += tail[i][3]; }
        if (tail[i][1] === "-") { result -= tail[i][3]; }
      }

      return result;
    }

Term
  = head:Factor tail:(_ ("*" / "/") _ Factor)* {
      var result = head, i;

      for (i = 0; i < tail.length; i++) {
        if (tail[i][1] === "*") { result *= tail[i][3]; }
        if (tail[i][1] === "/") { result /= tail[i][3]; }
      }

      return result;
    }

Factor
  = neg:NegId number:Factor_NUM { if (neg === "-") { return -number} return number }
  
Factor_NUM
  = "(" _ expr:Expression _ ")" { return expr; }
  / Integer

Integer "integer"
  = [0-9]+ { return parseInt(text(), 10); }
  
NegId
  = [\-+]? { return text()}

_ "whitespace"
  = [ \t\n\r]*