Description

One of the freelancer writers that is writing an article for the SEO-Hygiene initiative (Content Marketing & Growth Teams) got stuck on writing some sample code examples with Node.js driver for the example of using MongoDB mapReduce.
 
My team, Developer Advocate team, notice a couple things on the reference documentation that needs to be more clear:

• out needs to be an object: https://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html

• in JS in general array.sum does not look correct, it should be array.reduce

 

---

For Context the thread of the email of the freelancer writer:

At any rate, back to the issues with mapReduce; I attempted to run the following code:

"""
await collection.mapReduce(
        function() {
            emit(1, this.itemsTotal)
        },
        function(key, values) {
            return Array.sum(values)
        }
    )
"""

According to the Node driver's .d.ts files, this is valid usage. However, I got the error:

"""
TypeError: Cannot read property 'out' of undefined
    at Collection.mapReduce
"""

So, looking back at the docs page, I saw that there was a string assigned to "out":

"""
await collection.mapReduce(
        function() {
            emit(1, this.itemsTotal)
        },
        function(key, values) {
            return Array.sum(values)
        },
        {
            out: "Test"
        }
    )
"""

Not only did I get errors from typescript saying that "out" needed to be an object, I got the same error as before.

However, with some searching online, I found that you're able to change the last config object to:

"""
{
out: { inline: 1 }
}
"""

Once I did this, I got an array of values, without commas, not combined in any way, returned to the console.

Code:

"""
await collection.mapReduce(
        function() {
            emit(1, this.itemsTotal)
        },
        function(key, values) {
            return Array.sum(values);
        },
        {
            out: { inline: 1 }
        }
    )
"""

Output (shortened):

"""
[{"_id":1,"value":"NumberDecimal(\"1863.46\")NumberDecimal(\"798.25\")NumberDecimal(\"756.90\")NumberDecimal(\"758.22\")NumberDecimal(\"233.58\")NumberDecimal(\"122.05\")NumberDecimal(\"1628.90\")NumberDecimal(\"355.79\")NumberDecimal(\"1200.78\")NumberDecimal(\"719.72\")
"""

Not knowing how Array.sum works (it's not valid JS, but it's documented as the way to combine an array of numbers from the docs page), I decided to use `values.reduce` to manually add the numbers together:

"""
 await collection.mapReduce(
        function() {
            emit(1, this.itemsTotal)
        },
        function(key, values) {
            return values.reduce((p: number, c: any) => p + c, 0)
        },
        {
            out: { inline: 1 }
        }
    )
"""

However, I ended up just getting `0` as a string appended at the start

"""
[{"_id":1,"value":"0NumberDecimal(\"1863.46\")NumberDecimal(\"798.25\")NumberDecimal(\"756.90\")NumberDecimal(\"758.22\")NumberDecimal(\"233.58\")
"""

I then decided to try and get the value of `NumberDecimal`.

I tried using `parseInt` on `c`, which left me with `0` as a final result. I assume this is because `parseInt(Object)` is `NaN`

The only way I was able to get the results of the numbers combined was the following code:

"""
 await collection.mapReduce(
        function() {
            emit(1, this.itemsTotal)
        },
        function(key, values) {
            return values.reduce((p: number, c: any) => p + Number(JSON.parse(JSON.stringify(c))["$numberDecimal"], 0)
        },
        {
            out: { inline: 1 }
        }
    )
"""

However, this seems like an obvious anti-pattern and I wouldn't feel comfortable putting this as official docs that I wrote.

Just to make sure this wasn't a bug in the NodeJS driver, I attempted to use the Mongo Shell:

"""
use("sample_supplies");

db.sales.mapReduce(
    function() {
        emit(1, this.itemsTotal)
    },
    function(key, values) {
        return values.reduce((p, c) => p + c, 0)
    },
    {
        out: { inline: 1 }
    }
)
"""

And ran into the same issues as before (I tried all of the code variations I outlined for NodeJS as well with the same results)

 

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)