function lowercase( str ) {
	if ( !isString( str ) ) {
		throw new TypeError( format( 'invalid argument. Must provide a string. Value: `%s`.', str ) );
	}
	return base( str );
}
Converts a string to lowercase.

@param {string} str - string to convert
@throws {TypeError} must provide a string
@returns {string} lowercase string

@example
var str = lowercase( 'bEEp' );
// returns 'beep'