alternate to $strLenCP field in mongoDB 3.0[prior versions]

问题

I'm currently using mongo 3.0v. I've a requirement to find the length of each string in the result from aggregate command. For eg:

db.getCollection('temp').find()
[
 {"key": "value1"},
 {"key": "value2" },
 {"key": "valuee2"}
]

This query gives length of key field

db.getCollection('temp').aggregate([{
   $project: {
       "strLength": {"$strLenCP": "$key"}
   }
   }])

like

[
 {"strLength": 6},
 {"strLength": 6},
 {"strLength": 7}
]

But "$strLenCP" key is not supported in prior 3.4 versions. So is there any alternate options for this?

来源：https://stackoverflow.com/questions/51040884/alternate-to-strlencp-field-in-mongodb-3-0prior-versions