Mastering the map() and reduce() functions in JavaScript: A step-by-step guide


I have been working on a simple function to calculate prime numbers using a NoSQL database. Despite trying different approaches, I kept encountering an error stating that the value I was searching for is not defined. Any feedback or suggestions would be greatly appreciated.

Here's what I have attempted:

  1. First, I created a set of random numbers:
for (var i = 0; i < 1000; i++) db.exemplary.insert( {x: Math.random()} );

2. Next, I defined a map function

map = function() {
    for (var i = 2; i < x; i++){
        if(x % i == 0)
            try {} catch (e) {};
    }
        emit(i, value);
  };
  1. Then, I created the reduce function:
reduce = function (i,value){return array.sum(value)}
  1. After assigning the collection to a variable, I called the function:
collection = db.exemplary
var result = collection.mapReduce(map, reduce, {out: {inline: 1}});

Based on my basic understanding, the map step should verify if a number is prime or not, while the reduce step should add up all instances of prime numbers. The error I encountered mentioned that x is not defined, even though it is present in the db.exemplary collection.


I am open to any suggestions for improving the functionality described above.

EDIT: At the moment, my function is not "emitting" any results from the map operation. I am currently addressing this issue.

Answer №1

When using Math.random(), it will create floating-point numbers between 0 and less than 1. If you want to generate random numbers between 0 and 999, you can use a function like the one shown below:

{ x: Math.floor(Math.random()*1000) }

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