Behaviour of NumberLong in Mongo Shell
mongo-shell, mongodb
Solution
What's the difference between Number and NumberLong?
JavaScript currently only has a single numeric type which is Number, represented as an IEEE 754 double-precision floating point value (8 bytes).
MongoDB's BSON storage representation has more numeric types than JavaScript, including 32-bit signed integers (4 bytes) and 64-bit signed integers (8 bytes).
The `NumberInt()` and `NumberLong()` constructors are data types in the `mongo` shell that allow you to create integer values rather than using JavaScript's default floating point number. These are implemented as custom prototypes so the `typeof` these will be `object` (as opposed to `number` which is part of the JavaScript primitive types).
When you add `NumberLong()` or `NumberInt()` values together, JavaScript coerces the result into the native `number` type which is why your results in the 2nd and 4th test differ from the 1st and 3rd.
Why use NumberLong / NumberInt?
These types are used to provide an interface to MongoDB's underlying BSON storage format.
A 32-bit integer (`NumberInt`) can be represented in half the bytes as compared to a `Number` or `NumberLong`.
A 64-bit integer (`NumberLong`) has more precision for large integer values as compared to a `Number`. Since a double-precision floating point representation reserves some bits for the exponent, the largest `Number` that can be precisely stored is 253 versus 263-1 for a `NumberLong`.
Problem
I wrote the following 4 statements in the Mongo Shell i) `NumberLong(3)` ii) `NumberLong(3)+NumberLong(4)` iii) `typeof NumberLong(3)` iv) `typeof (NumberLong(3)+NumberLong(4))` and their corresponding outputs were i) `NumberLong(3)` ii) `7` iii) `object` iv) `number` Although the second result makes the fourth one obvious, I am not able to get to the head or tail of this behaviour.What is happening behind the scenes?? I tried finding the underlying concept in the MongoDB documentation but couldn't find much. Please Help!!