4.1 primitive and reference values

　　ECMAScript variables may contains two different types of data: primitive values and reference values. Primitive values are simple atomic pieces of data, while reference values are objects that may be made up of multiple values.

　　The five primitive types are: Undefined, Null, Boolean, Number and String.

　　primitive values are accessed by value, while reference values are accessed by reference.

Copying Values

　　Primitive and reference values act differently when copied from one variable to another.

　　When a primitive value is assigned from one variable to another, the value stored on the variable object is created and copied into the location for the new variable.

　　When a reference value is assigned from one variable to another, the value stored on the variable object is also copied into the location for the new variable. The different is that this value is actually a pointer to an object stored on the heap. Once the operation is complete, two variables point to exactly the same object, so changes to one are reflected on the other.

Argument Passing

　　When an argument is passed by value, the value is copied into a local variable(a named argument and, in ECMAScript, a slot in the arguments object). When an argument is passed by reference, the location of a value in memory is stored into a local variable, which means that changes to the local variable are reflected outside of the function .