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[PART 1] Useful JavaScript Tips, Tricks and Best Practices


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Note that the code snippets in this article have been tested in the latest Google Chrome version 30, which uses the V8 JavaScript Engine (V8

1 – Don’t forget var keyword when assigning a variable’s value for the first time.
Assignment to an undeclared variable automatically results in a global variable being created. Avoid global variables.

2 – use === instead of ==
The == (or !=) operator performs an automatic type conversion if needed. The === (or !==) operator will not perform any conversion. It compares the value and the type, which could be considered faster than ==.

[10] === 10 // is false
[10] == 10 // is true
'10' == 10 // is true
'10' === 10 // is false
[] == 0 // is true
[] === 0 // is false
'' == false // is true but true == "a" is false
'' === false // is false
3 – undefined, null, 0, false, NaN, '' (empty string) are all falsy.

4 – Use Semicolons for line termination

The use of semi-colons for line termination is a good practice. You won’t be warned if you forget it, because in most cases it will be inserted by the JavaScript parser. For more details about why you should use semi-colons, take a look to this artice: http://davidwalsh.name/javascript-semicolons.

5 – Create an object constructor

function Person(firstName, lastName){
this.firstName = firstName;
this.lastName = lastName;

var Saad = new Person("Saad", "Mousliki");
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6 – Be careful when using typeof, instanceof and constructor.
  • typeof : a JavaScript unary operator used to return a string that represents the primitive type of a variable, don’t forget that typeof null will return “object”, and for the majority of object types (Array, Date, and others) will return also “object”.
  • constructor : is a property of the internal prototype property, which could be overridden by code.
  • instanceof : is another JavaScript operator that check in all the prototypes chain the constructor it returns true if it’s found and false if not.
var arr = ["a", "b", "c"];
typeof arr; // return "object"
arr instanceof Array // true
arr.constructor(); //[]

7 – Create a Self-calling Function
This is often called a Self-Invoked Anonymous Function or Immediately Invoked Function Expression (IIFE). It is a function that executes automatically when you create it, and has the following form:

// some private code that will be executed automatically
var result = a+b;
return result;

8 – Get a random item from an array

var items = [12, 548 , 'a' , 2 , 5478 , 'foo' , 8852, , 'Doe' , 2145 , 119];

var randomItem = items[Math.floor(Math.random() * items.length)];

9 – Get a random number in a specific range

This code snippet can be useful when trying to generate fake data for testing purposes, such as a salary between min and max.

var x = Math.floor(Math.random() * (max - min + 1)) + min;

10 – Generate an array of numbers with numbers from 0 to max

var numbersArray = [] , max = 100;

for( var i=1; numbersArray.push(i++) < max;); // numbers = [1,2,3 ... 100]