javascript

Variables, Scope,
and Memory
The nature of variables in JavaScript, as defined in ECMA - 262 Third Edition, is quite unique
compared to other languages. Being loosely typed, a variable is literally just a name for a particular
value at a particular time. Because there are no rules defining the type of data that a variable must
hold, a variable ’ s value and data type can change during the lifetime of a script. Though this is an
interesting, powerful, and problematic feature, there are many more complexities related to
variables.
Primitive and Reference Values
ECMAScript variables may contain two different types of data: primitive values and reference
values. Primitive values are simple pieces of data that are stored in memory on the stack , which is to
say that the value is completely stored in one memory location. Reference values , on the other hand,
are objects that are stored on the heap , meaning that the value stored in the variable is actually just
a pointer to another memory location where the object is stored.
When a value is assigned to a variable, the interpreter must determine if it ’ s a primitive or a
reference. The five primitive types were discussed in the previous chapter: Undefined, Null,
Boolean, Number, and String. Each of these data types takes up a fixed amount of space, so values
can easily be stored on the stack. Doing so also allows for fast variable lookup. These variables are
said to be accessed by value , because you are manipulating the actual value stored in the variable.
In many languages, strings are represented by objects and are therefore considered to be reference
types. ECMAScript breaks away from this tradition.
If a reference value is assigned to a variable, space must be allocated on the heap. Reference values
cannot be stored on the stack, because they don ’ t have fixed sizes. A memory address does