Avoid global variables, avoid new, avoid ==, avoid eval()
Minimize the use of global variables.
This includes all data types, objects, and functions.
Global variables and functions can be overwritten by other scripts.
Use local variables instead, and learn how to use closures.
All variables used in a function should be declared as local variables.
Local variables must be declared with the var keyword, otherwise they will become global variables.
|Strict mode does not allow undeclared variables.|
It is good coding practice, to put all declarations at the top of each script or function. This makes it easier to avoid unwanted (implied) global variables. It also gives cleaner code, and reduces the possibility of unwanted re-declarations.
Variable declarations should always be the first statements in scripts and functions.
This also goes for variables in loops:
Always treat numbers, strings, or booleans as primitive values. Not as objects.
Declaring numbers, strings, or booleans as objects, slows down execution speed, and produces nasty side effects:
Beware that numbers can accidentally be converted to strings or NaN (Not a Number).
Subtracting a string from a string, does not generate an error but returns NaN (Not a Number):
The == comparison operator always converts (to matching types) before comparison.
The === operator forces comparison of values and type:
If a function is called with a missing argument, the value of the missing argument is set to undefined.
Undefined values can break your code. It is a good habit to assign default values to arguments.
Or, even simpler:
Read more about function parameters and arguments at Function Parameters
The eval() function is used to run text as code. In almost all cases, it should not be necessary to use it.
Because it allows arbitrary code to be run, it also represents a security problem.