MongoDB Built function __ function

To define a function:

Db.system.js.insert ({
        _id: "Testconcat", 
      value:function testconcat (S1, S2) {
                   return s1 + s2
              }
   });

To run the function:

Db.eval (' Testconcat ("abc", "123");

Results:

abc123

The prerequisite is that you must have permission to execute the eval function, as detailed in this article:

http://blog.csdn.net/yabingshi_tech/article/details/47660493

--Note that if you define a function directly using the following similar form, after exiting the session, log in again, call the function, and discover that the function disappears.

Example:

> Function Testconcat (S1, s2) {... return s1 + s2
...               }
> Testconcat (2,3)

5

Exit

> Testconcat (2,3);
2014-02-19t14:11:10.499+0800 Referenceerror:testconcat is not defined

--This article is mainly referred to from: http://blog.csdn.net/miyatang/article/details/19486797,http://www.2cto.com/database/201503/382391.html.