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JavaScript Guideline

Variables, constants and function names

  • Variable and function names must always be written in camelCase.
  • All variable and constant names must start with a letter.
  • Underscores in variable and constant names may be used when necessary.
  • Hyphens must not be used.
  • All local variables in a function must be declared with the var keyword.
  • Global variables and constants must be written in UPPERCASE.
  • The use of global variables should be minimized.
var firstName = 'John';
var lastName = 'Doe';

Operators

  • Always put spaces around operators ( = + - * / ), and after commas:
  • Calculations should be used in brackets.
var x = (y + z);
var values = ["Volvo", "Saab", "Fiat"];

Statements

  • Statements must end with a semicolon
var name = "John Doe";

Complex statements

  • Complex statements must always use brackets.
  • The opening bracket must be set at the end of the first line.
  • Before the opening bracket one space must be set.
  • The closing bracket must be set on a new line, without leading spaces.
  • Do not end a complex statement with a semicolon.

Functions:

function toCelsius(fahrenheit) {
    return (5 / 9) * (fahrenheit - 32);
}

Loops:

for (i = 0; i < 5; i++) {
    x += i;
}

Conditionals:

if (time < 20) {
    greeting = "Good day";
} else {
    greeting = "Good evening";
}

Objects

  • The opening bracket must be placed on the same line as the object name.
  • Use colon plus one space between each property and its value.
  • Quotes must be placed around string values and must not be placed around numeric or boolean values.
  • Do not add a comma after the last property-value pair.
  • Place the closing bracket on a new line, without leading spaces.
  • Always end an object definition with a semicolon.
  • Short objects can be written in one line, using spaces only between properties.
var person = {
    firstName: "John",
    lastName: "Doe",
    age: 50,
    eyeColor: "blue"
};
var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
  • Never declare Number, String, or Boolean Objects
var x = "John";             
var y = new String("John");
(x === y) // is false because x is a string and y is an object.
var x = new String("John");             
var y = new String("John");
(x == y) // is false because you cannot compare objects.

Declarations

  • Declarations must be placed on top
// Declare at the beginning
var firstName, lastName, price, discount, fullPrice;

// Use later
firstName = "John";
lastName = "Doe";

price = 19.90;
discount = 0.10;

fullPrice = price * 100 / discount;

Do Not use new Object()

  • Use {} instead of new Object()
  • Use "" instead of new String()
  • Use 0 instead of new Number()
  • Use false instead of new Boolean()
  • Use [] instead of new Array()
  • Use /()/ instead of new RegExp()
  • Use function (){} instead of new Function()

Comparison

  • Comparison with value and type === is strongly recommended.
"1" === 1 // false
1 === 1   // true

Security

  • eval() must be avoided, if possible.

Sources