parseInt()

Takes two parameters: an input object and radix; then tries to return an integer representation of the input. Doesn't handle exponents in the input. The default radix is 10 (a decimal number). Returns NaN on failure. Omitting the radix may lead to unexpected results (for example for inputs such as 08), so it's best to always specify it.

> parseInt('10e+3');
10
> parseInt('FF');
NaN
> parseInt('FF', 16);
255

parseFloat()

Takes a parameter and tries to return a floating-point number representation of it. Understands exponents in the input.

> parseFloat('10e+3');
10000
> parseFloat('123.456test');
123.456

isNaN()

Abbreviated from "Is Not a Number". Accepts a parameter and returns true if the parameter is not a valid number, false otherwise. Attempts to convert the input to a number first.

> isNaN(NaN);
true
> isNaN(123);
false
> isNaN(parseInt('FF'));
true
> isNaN(parseInt('FF', 16));
false

isFinite()

Returns true if the input is a number (or can be converted to a number), but is not the number Infinity or -Infinity. Returns false for infinity or non-numeric values.

> isFinite(1e+1000);
false
> isFinite(-Infinity);
false
> isFinite("123");
true

encodeURIComponent()

Converts the input into a URL-encoded string. For more details on how URL encoding works, refer to the Wikipedia article at http://en.wikipedia.org/wiki/Url_encode.

> encodeURIComponent('http://phpied.com/');
"http%3A%2F%2Fphpied.com%2F"
> encodeURIComponent('some script?key=v@lue');
"some%20script%3Fkey%3Dv%40lue"

decodeURIComponent()

Takes a URL-encoded string and decodes it.

> decodeURIComponent('%20%40%20');
" @ "

encodeURI()

URL-encodes the input, but assumes a full URL is given, so returns a valid URL by not encoding the protocol (for example, http://) and hostname (for example, www.phpied.com).

> encodeURI('http://phpied.com/');
"http://phpied.com/"
> encodeURI('some script?key=v@lue');
"some%20script?key=v@lue"

decodeURI()

Opposite of encodeURI().

> decodeURI("some%20script?key=v@lue");
"some script?key=v@lue"

eval()

Accepts a string of JavaScript code and executes it. Returns the result of the last expression in the input string.

To be avoided where possible.

> eval('1 + 2');
3
> eval('parseInt("123")');
123
> eval('new Array(1, 2, 3)');
[1, 2, 3]
> eval('new Array(1, 2, 3); 1 + 2;');
3