Avoid global variables, avoid
new, avoid==, avoideval()
Avoid Global Variables #
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.
Always Declare Local Variables #
All variables used in a function should be declared as local variables.
Local variables must be declared with the var keyword or the let keyword, otherwise they will become global variables.
Strict mode does not allow undeclared variables.
Declarations on Top #
It is a good coding practice to put all declarations at the top of each script or function.
This will:
- Give cleaner code
- Provide a single place to look for local variables
- Make it easier to avoid unwanted (implied) global variables
- Reduce the possibility of unwanted re-declarations
// Declare at the beginning
var firstName, lastName, price, discount, fullPrice;
// Use later
firstName = "John";
lastName = "Doe";
price = 19.90;
discount = 0.10;
fullPrice = price - discount; This also goes for loop variables:
// Declare at the beginning
var i;
// Use later
for (i = 0; i < 5; i++) { By default, JavaScript moves all declarations to the top (JavaScript Hoisting).
Initialize Variables #
It is a good coding practice to initialize variables when you declare them.
This will:
- Give cleaner code
- Provide a single place to initialize variables
- Avoid undefined values
// Declare and initiate at the beginning
var firstName = "",
lastName = "",
price = 0,
discount = 0,
fullPrice = 0,
myArray = [],
myObject = {}; Initializing variables provides an idea of the intended use (and intended data type).
Never Declare Number, String, or Boolean Objects #
Always treat numbers, strings, or booleans as primitive values. Not as objects.
Declaring these types as objects, slows down execution speed, and produces nasty side effects:
Example #
var x = "John";
var y = new String("John");
(x === y) // is false because x is a string and y is an object. Or even worse:
Example #
var x = new String("John");
var y = new String("John");
(x == y) // is false because you cannot compare objects. var x1 = {}; // new object
var x2 = ""; // new primitive string
var x3 = 0; // new primitive number
var x4 = false; // new primitive boolean
var x5 = []; // new array object
var x6 = /()/; // new regexp object
var x7 = function(){}; // new function object var x = "Hello"; // typeof x is a string
x = 5; // changes typeof x to a number When doing mathematical operations, JavaScript can convert numbers to strings:
Example #
var x = 5 + 7; // x.valueOf() is 12, typeof x is a number
var x = 5 + "7"; // x.valueOf() is 57, typeof x is a string
var x = "5" + 7; // x.valueOf() is 57, typeof x is a string
var x = 5 - 7; // x.valueOf() is -2, typeof x is a number
var x = 5 - "7"; // x.valueOf() is -2, typeof x is a number
var x = "5" - 7; // x.valueOf() is -2, typeof x is a number
var x = 5 - "x"; // x.valueOf() is NaN, typeof x is a number
Subtracting a string from a string, does not generate an error but returns NaN (Not a Number):
Example #
"Hello" - "Dolly" // returns NaN 0 == ""; // true
1 == "1"; // true
1 == true; // true
0 === ""; // false
1 === "1"; // false
1 === true; // false function myFunction(x, y) {
if (y === undefined) {
y = 0;
}
} ECMAScript 2015 allows default parameters in the function definition:
function (a=1, b=1) { /*function code*/ } Read more about function parameters and arguments at Function Parameters
End Your Switches with Defaults #
Always end your switch statements with a default. Even if you think there is no need for it.
Example #
switch (new Date().getDay()) {
case 0:
day = "Sunday";
break;
case 1:
day = "Monday";
break;
case 2:
day = "Tuesday";
break;
case 3:
day = "Wednesday";
break;
case 4:
day = "Thursday";
break;
case 5:
day = "Friday";
break;
case 6:
day = "Saturday";
break;
default:
day = "Unknown";
} Avoid Using eval() #
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.
