JavaScrip Tricks

Variable Scope

In many of programming languages we see that the scope of the variables are block level. where as in JavaScript the scope of variables are function level. Below is the Example.

Example 1:

if (true) {

var a = 10;

}

console.log('a = ', a);

Result

a = 10

Example 2:

function : someFunction(){

var a = 10;

}

console.log('a = ', a);

Result

a = undefined

Infinity

There is a special value in JavaScript called Infinity. It represents a number too big for JavaScript to handle.

Infinity is indeed a number, as typing typeof Infinity in the console will confirm.

Typeof Infinity

number

Dividing by 0 will give you infinity.

var a = 6 / 0;

console.log(a);

Result

Infinity

there's no practical value to it.

Infinity - Infinity

NaN

-Infinity + Infinity

NaN

Any other arithmetic operation with Infinity as one of the operands will give you Infinity:

Infinity - 10

Infinity

-Infinity * 2

-Infinity

Infinity / 3

Infinity

Infinity - 99999999999999999

Infinity

NaN (Not A Number)

NaN is a special value that is also a number

typeof NaN

"number"

var a = NaN;

a

NaN

You get NaN when you try to perform an operation that assumes numbers but the operation fails. For example, if you try to multiply 10 by the character "f", the result is NaN,

var a = 10 * "f";

a

NaN

Checking if a Variable Exists

The laziest way to do this is simply putting the variable in the condition part of the if, for example if (somevar) {...}, but this is not necessarily the best method. Let's take a look at an example that tests whether a variable called somevar exists, and if so, sets the result variable to yes:

var result = '';

if (somevar){result = 'yes';}

somevar is not defined

Result

""

This code obviously works, because at the end result was not "yes". But firstly, the code generated a warning: somevar is not defined and as a JavaScript whiz you don't want your code to do anything like that. Secondly, just because if (somevar) returned false doesn't mean that somevar is not defined. It could be that somevar is defined and initialized but contains a falsy value, like false or 0.

A better way to check if a variable is defined is to use typeof.

if (typeof somevar !== "undefined"){result = 'yes';}

result;

""

typeof will always return a string and you can compare this string with "undefined". Note that the variable somevar may have been declared but not assigned a value yet and you'll still get the same result. So when testing with typeof like this, you're really testing whether the variable has any value (other than the value undefined).

var somevar;

if (typeof somevar !== "undefined"){result = 'yes';}

result;

""

somevar = undefined;

if (typeof somevar !== "undefined"){result = 'yes';}

result;

""

If a variable is defined and initialized with any value other than undefined, its type returned by typeof is no longer "undefined".

somevar = 123;

if (typeof somevar !== "undefined"){result = 'yes';}

result;

"yes"

String Conversions

When you use a number-like string as an operand in an arithmetic operation, the string is converted to a number behind the scenes. (This works for all operations except addition, because of addition's ambiguity)

var s = '1';

typeof s

“String”

s = 3 * s;

typeof s;

"number"

s

3

var s = '1'; s++; typeof s;

"number"

s

2

A lazy way to convert any number-like string to a number is to multiply it by 1 (a better way is to use a function called parseInt() )

If the conversion fails, you'll get NaN:

var d = '101 a';

d * 1

NaN

A lazy way to convert anything to a string is to concatenate it with an empty string.

var n = 1;

typeof n;

"number"

n = "" + n;

"1"

typeof n;

"string"

Best Practice Tips

Minimize the number of global variables.

Imagine two people working on two different functions in the same script and they both decide to use the same name for their global variable. This could easily lead to unexpected results and hard-to-find bugs.

Always declare your variables with the var statement.

Here's an interesting example that shows an important aspect of the local versus global scoping.

var a = 123;

function f() {

alert(a);

var a = 1;

alert(a);

}

f();

The first alert will show "undefined". This is because inside the function the local scope is more important than the global scope. So a local variable overwrites any global variable with the same name. At the time of the first alert() a was not yet defined (hence the value undefined) but it still existed in the local space.