[SERVER-37690] countDocuments throws an error when matching 0 document - MongoDB Jira

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Backwards Compatibility:
Minor Change
Operating System:
ALL
Backport Requested:

v4.2, v4.0
Steps To Reproduce:

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```

use count

db.count.insert([\{a: 1}, \{a: 2}])

db.count.count({a:1}) // returns 1

db.count.count({a:3}) // returns 0

db.count.countDocuments({a:1}) // returns 1

db.count.countDocuments({a:3}) // throws an exception

```

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``` use count db.count.insert( [\{a: 1}, \{a: 2}] ) db.count.count({a:1}) // returns 1 db.count.count({a:3}) // returns 0 db.count.countDocuments({a:1}) // returns 1 db.count.countDocuments({a:3}) // throws an exception ```

`countDocuments` throws an error when matching 0 document.

I would have expected it to follow what `count` does, which is return 0.

With both versions `4.0.3` and `4.1.4` (haven't checked intermediary ones), the error I get from the bellow `steps to reproduce` is:

2018-10-20T16:26:44.252+0100 E QUERY [js] TypeError: res[0] is undefined :

 DBCollection.prototype.countDocuments@src/mongo/shell/collection.js:1418:5

 @(shell):1:1

is documented by

DOCS-12966 Investigate changes in SERVER-37690: countDocuments throws an error when matching 0 document

is related to

SERVER-40718 Validate parameters for estimatedDocumentCount

Participants:: A. Jesse Jiryu Davis, David Storch, Githook User, Miha Filej, Ravind Kumar, Xavier GUIHOT