Basic calculator http://jsbin.com/nowela/edit?html,js

calc.js

    function calcFromStr(str) {
        var ops = ['*','/','+','-'];
        var reg = /(\b\D)/;
        var calc = str.split(reg);
      
        function calculator(l, op, r) {
          
          switch( op ) {
            case '*':
              return l*r;
            case '/':
              return l/r;
            case '+':
              return l+r;
            case '-':
              return l-r;
              
          }
          
        }
      
        calc.forEach(function(el, ix) {
          
          if ( ix % 2 === 0 ) {
            
            calc[ix] = Number(el);
          }
        });
        
        do {
          var op = ops.shift();
          var ix;
          var nval;
          

          do {
            ix = calc.indexOf(op);
            
            if ( ix > 0 ) {
              
              nval = calculator(calc[ix - 1], op, calc[ix + 1]);
              calc.splice(ix-1, 3, nval);
            }
            
          } while ( ix > -1 );
          
        } while( ops.length );
      
        return calc[0];
    }