src/compile/expression.js
function(ast){

      var exp = ast.expression;
      var ret;
      if (ast.type === 'math') {

        switch(ast.operator) {
          case '+':
          ret = this.getExpression(exp[0]) + this.getExpression(exp[1]);
          break;

          case '-':
          ret = this.getExpression(exp[0]) - this.getExpression(exp[1]);
          break;

          case '/':
          ret = this.getExpression(exp[0]) / this.getExpression(exp[1]);
          break;

          case '%':
          ret = this.getExpression(exp[0]) % this.getExpression(exp[1]);
          break;

          case '*':
          ret = this.getExpression(exp[0]) * this.getExpression(exp[1]);
          break;

          case '||':
          ret = this.getExpression(exp[0]) || this.getExpression(exp[1]);
          break;

          case '&&':
          ret = this.getExpression(exp[0]) && this.getExpression(exp[1]);
          break;

          case '>':
          ret = this.getExpression(exp[0]) > this.getExpression(exp[1]);
          break;

          case '<':
          ret = this.getExpression(exp[0]) < this.getExpression(exp[1]);
          break;

          case '==':
          ret = this.getExpression(exp[0]) == this.getExpression(exp[1]);
          break;

          case '>=':
          ret = this.getExpression(exp[0]) >= this.getExpression(exp[1]);
          break;

          case '<=':
          ret = this.getExpression(exp[0]) <= this.getExpression(exp[1]);
          break;

          case '!=':
          ret = this.getExpression(exp[0]) != this.getExpression(exp[1]);
          break;

          case 'minus':
          ret = - this.getExpression(exp[0]);
          break;

          case 'not':
          ret = !this.getExpression(exp[0]);
          break;

          case 'parenthesis':
          ret = this.getExpression(exp[0]);
          break;

          default:
          return;
          // code
        }

        return ret;
      } else {
        return this.getLiteral(ast);
      }
    }
function(ast){

      var exp = ast.expression;
      var ret;
      if (ast.type === 'math') {

        switch(ast.operator) {
          case '+':
          ret = this.getExpression(exp[0]) + this.getExpression(exp[1]);
          break;

          case '-':
          ret = this.getExpression(exp[0]) - this.getExpression(exp[1]);
          break;

          case '/':
          ret = this.getExpression(exp[0]) / this.getExpression(exp[1]);
          break;

          case '%':
          ret = this.getExpression(exp[0]) % this.getExpression(exp[1]);
          break;

          case '*':
          ret = this.getExpression(exp[0]) * this.getExpression(exp[1]);
          break;

          case '||':
          ret = this.getExpression(exp[0]) || this.getExpression(exp[1]);
          break;

          case '&&':
          ret = this.getExpression(exp[0]) && this.getExpression(exp[1]);
          break;

          case '>':
          ret = this.getExpression(exp[0]) > this.getExpression(exp[1]);
          break;

          case '<':
          ret = this.getExpression(exp[0]) < this.getExpression(exp[1]);
          break;

          case '==':
          ret = this.getExpression(exp[0]) == this.getExpression(exp[1]);
          break;

          case '>=':
          ret = this.getExpression(exp[0]) >= this.getExpression(exp[1]);
          break;

          case '<=':
          ret = this.getExpression(exp[0]) <= this.getExpression(exp[1]);
          break;

          case '!=':
          ret = this.getExpression(exp[0]) != this.getExpression(exp[1]);
          break;

          case 'minus':
          ret = - this.getExpression(exp[0]);
          break;

          case 'not':
          ret = !this.getExpression(exp[0]);
          break;

          case 'parenthesis':
          ret = this.getExpression(exp[0]);
          break;

          default:
          return;
          // code
        }

        return ret;
      } else {
        return this.getLiteral(ast);
      }
    }
表达式求值，表达式主要是数学表达式，逻辑运算和比较运算，到最底层数据结构，
基本数据类型，使用 getLiteral求值，getLiteral遇到是引用的时候，使用
getReferences求值