JSTypes is a small JavaScript library that does two things:

1. Check if variables are of certain types (numbers, strings, undefined)
2. Reliably create certain values (undefined, NaN, Infinity)

Note that you probably shouldn't use JSTypes unless you need it -- other popular libraries like jQuery and Underscore already have many of these methods. If you aren't using these libraries, then JSTypes might be of help!

Examples:

var x = 1;
var y = new Number(22);
x instanceof Number;    // false; not what we want!
JSTypes.isNumber(x);    // true
JSTypes.isNumber(y);    // true

var x;
JSTypes.isUndefined(x);    // true
undefined = 10;            // this is terrible code, but it might get written!
JSTypes.isUndefined(x);    // still true

var x = 5;
undefined = 10;
x = undefined;                  // bad: x = 10
x = JSTypes.makeUndefined();    // good: x is now really undefined
JSTypes.isUndefined(x);         // true

I recommend (and do) move this library into your code's namespace if you're writing a library. Just replace all occurrences of JSTypes with your library's namespace.

For licensing info, see LICENSE.txt.

Here's everything in the API.

• JSTypes.isNumber(toCheck) returns true if something is a number
• JSTypes.isInteger(toCheck) returns true if something is an integer
• JSTypes.isString(toCheck) returns true if something is a string
• JSTypes.isBoolean(toCheck) returns true if something is a boolean
• JSTypes.isArray(toCheck) returns true if something is an array
• JSTypes.isDefined(toCheck) returns true if something is defined (will return false for undefined, or null, or NaN)
• JSTypes.isUndefined(toCheck) returns true if something is undefined
• JSTypes.isNAN(toCheck) returns true if something is NaN
• JSTypes.isInfinite(toCheck) returns true of something is positive or negative infinity
• JSTypes.makeUndefined() always returns undefined, even if it's stupidly redefined
• JSTypes.makeNaN() always returns NaN, even if it's stupidly redefined
• JSTypes.makeInfinity() always returns Infinity, even if it's stupidly redefined