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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 or the let keyword,or the const keyword,
otherwise they will become global variables.
Strict mode does not allow undeclared variables.
It is a good coding practice to put all declarations at the top of each script or function.
This will:
// Declare at the beginning
let 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:
for (let i = 0; i < 5; i++)
{
It is a good coding practice to initialize variables when you declare them.
This will:
// Declare and initiate at the beginning
let firstName = "";
let lastName = "";
let price = 0;
let discount = 0;
let fullPrice = 0,
const myArray = [];
const myObject = {};
Initializing variables provides an idea of the intended use (and intended data type).
Declaring objects with const will prevent any accidental change of type:
let car = {type:"Fiat", model:"500", color:"white"};
car = "Fiat"; // Changes object to string
const car = {type:"Fiat", model:"500", color:"white"};
car = "Fiat"; // Not possible
Declaring arrays with const will prevent any accidential change of type:
let cars = ["Saab", "Volvo", "BMW"];
cars = 3; // Changes array to number
const cars = ["Saab", "Volvo", "BMW"];
cars = 3; // Not possible
"" instead of new String()0 instead of new Number()false instead of new Boolean(){} instead of new Object()[] instead of new Array()/()/ instead of new RegExp()function (){} instead of new Function()
let x1 = ""; // new primitive string
let x2 = 0; // new primitive number
let x3 = false; // new primitive boolean
const x4 = {}; // new object
const x5 = []; // new array object
const x6 = /()/; // new regexp object
const x7 = function(){}; // new function object
JavaScript is loosely typed.
A variable can contain all data types.
A variable can change its data type:
let x = "Hello"; // typeof x is a string
x = 5; // changes typeof x to a number
Beware that numbers can accidentally be converted to strings or NaN (Not a
Number).
When doing mathematical operations, JavaScript can convert numbers to strings:
let x = 5 + 7; // x.valueOf() is 12, typeof x is a number
let x = 5 + "7"; // x.valueOf() is 57, typeof x is a string
let x = "5" + 7; // x.valueOf() is 57, typeof x is a string
let x = 5 - 7; // x.valueOf() is -2, typeof x is a number
let x = 5 - "7"; // x.valueOf() is -2, typeof x is a number
let x = "5" - 7; // x.valueOf() is -2, typeof x is a number
let 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):
"Hello" - "Dolly" // returns NaN
The == comparison operator always converts (to matching types) before
comparison.
The === operator forces comparison of values and type:
0 == ""; // true
1 == "1"; // true
1 == true; // true
0 === ""; // false
1 === "1"; // false
1 === true;
// false
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.
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
Always end your switch statements with a default. Even if you think there is
no need for it.
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";
}
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:
let x = "John";
let y = new String("John");
(x === y) // is false because x is a string and y is an object.
Or even worse:
let x = new String("John");
let y = new String("John");
(x == y) // is false because you cannot compare objects.
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.