Differences between basic types and reference types in JavaScript

Code Listing 1:
var a=3.14;
var b=a;
a=4;
alert (b);

Code 2
var a=[1,2,3];
var b=a;
a[0]=99;
alert (b);

Why does code 1 show 3.14, why does the value of a not change? Obviously, after executing the second line of code, we re-assigned a value. Is it because the value of the first row given to a is an underlying type that cannot be changed?
That's why code 2 shows the value is 99,2,3, why the value of a is changed, because the array is a reference type, the reference type does not have a fixed size, it does not really assign to whom, but just assigns a value to B a reference, tell the variable b where to find it, is that it? Can you change the value of a reference type to a primitive type?
Who can give me a plain explanation! Thank you.

Differences between basic types and reference types in JavaScript