Question

The following code:

var r = /^[0-9A-Z]$/.test;
r("A")

Throws ‘TypeError: can’t convert undefined to object’

How else could I assign the test function to a variable, for passing in functions, later evaluation, etc.? (Without wrapping the regex in another function)

Update:

Consider this bit of valid code before answering:

var o = { f: function() { return 1 } };
var a = o.f;
var b = a();     // b = 1

Answer 1

It has to do with the value of this inside the test method.

For example:

var obj = {
  method: function () { return this === obj }
};

obj.method(); // true

var method = obj.method;
method(); // false

If you call the test method “as a function” -as your example r();-, the this value will refer to undefined (for built-in or strict functions in ECMAScript 5, in the above example this will refer to the global object).

Calling any method of RegExp.prototype with a this value that is not a RegExp object, will always generate this TypeError exception, quoting the spec:

15.10.6 Properties of the RegExp Prototype Object

In the following descriptions of functions that are properties of the RegExp prototype object, the phrase “this RegExp object” refers to the object that is the this value for the invocation of the function; a TypeError exception is thrown if the this value is not an object or an object for which the value of the [[Class]] internal property is not “RegExp”.

However you could bind the test method to your r function, using the Function.prototype.bind method:

var re = /^[0-9A-Z]$/,
    r = re.test.bind(re);

r("A"); // true

Or using call or apply:

var r = re.test;
r.call(re, "A"); // true