function isDigitString( x ) {
	var len;
	var ch;
	var i;
	if ( !isString( x ) ) {
		return false;
	}
	len = x.length;
	if ( len === 0 ) {
		return false;
	}
	for ( i = 0; i < len; i++ ) {
		ch = x.charCodeAt( i );
		if ( ch < ZERO || ch > NINE ) {
			return false;
		}
	}
	return true;
}
Tests whether a string contains only numeric digits.

@param {*} x - value to test
@returns {boolean} boolean indicating if a string contains only numeric digits

@example
var out = isDigitString( '0123456789' );
// returns true

@example
var out = isDigitString( '0xffffff' );
// returns false

@example
var out = isDigitString( '' );
// returns false

@example
var out = isDigitString( 123 );
// returns false