All Variabes Hold References

All variables in JavaScript hold references to values in memory. In lower level languages primitive values like numbers and booleans are stored in variables directly. In JavaScript primitives like numbers and booleans are not values held in variables themselves. Numbers and booleans are stored strictly in the heap and the variables which represent those primitive data types have references to those variables in memory. This is why strings can be considered primitive types in JavaScript even though they are really arrays under the hood.

The Map is not the Territory

Sometimes the most accurate perception of something is not the most effective perception of something. Human's have developed heuristics to allow for quick decision making. If we did not use heuristics then we would have to go through a lengthy decision making process for simple tasks like opening a door. In programming human's also use heuristics. For example, programmers believe that primitive values are singletons. Although this is not really true it is a useful way to think of it in the context of programming.

The Map: Only a Single 9 Exist

All instances of primitive values in JavaScript are singletons. Only a single instance of a given primitive value exist in memory at any given time. It appears as though that if you were to instantiate a number, then that number is created in memory. Any other variables that reinstantiate that number, will actually be referencing the same number in memory that was already created.

var a = 9;
var b = 9;
a === b
>>>true

On the surface, There can only be single number 9 in JavaScript; although this is not true in actuality it is a good way to think about it.

The Territory: There are Multiple 9's

The truth is that although the notion that only a single 9 can exist in memory at a time it is a good mental model for programming, it is not technically accurate. JavaScript is optimized in such a way as such multiple 9 values can exist in memory. JavaScript does black magic under the hood to give the illusion there is only one 9 value in memory.