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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

Strict mode

The "use strict"; directive was new in ECMAScript 5 and will be ignored by earlier versions of JavaScript. With the strict mode enabled, you can not, for example, use undeclared variables.

The strict mode should be used wherever possible as it makes it easier to write "secure" JavaScript and previously accepted "bad syntax" turns into real errors.

Security

  • eval() must be avoided, if possible.

Sources