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This tutorials guide you to write better code.
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, the let, or the const keyword, otherwise they will become global variables.
Global variables and functions can be overwritten by other scripts.
Use local variables instead, and learn how to use closures.
It is a good coding practice to put all declarations at the top of each script or function. This also goes for loop variables.
// 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;
for (let i = 0; i < 5; i++) {}
It is a good coding practice to initialize variables when you declare them. Initializing variables provides an idea of the intended use (and intended data type).
// Declare and initiate at the beginning
let firstName = "";
let lastName = "";
let price = 0;
let discount = 0;
let fullPrice = 0,
const myArray = [];
const myObject = {};
Declaring arrays or 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
let cars = ["Saab", "Volvo", "BMW"];
cars = 3; // Changes array to number
const cars = ["Saab", "Volvo", "BMW"];
cars = 3; // Not possible
It is a good coding practice to initialize variables when you declare them. Initializing variables provides an idea of the intended use (and intended data type).
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
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;
}
}
function (a=1, b=1) { /*function code*/ }
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.
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