index.js

/* eslint-disable no-restricted-syntax, func-names, no-empty-function */
'use strict';

var RE_FUNCTION_NAME = require( './../lib' );

function fname( fcn ) {
	return RE_FUNCTION_NAME.exec( fcn.toString() )[ 1 ];
}

console.log( fname( Math.sqrt ) );
// => 'sqrt'

console.log( fname( Int8Array ) );
// => 'Int8Array'

console.log( fname( Object.prototype.toString ) );
// => 'toString'

console.log( fname( function () {} ) );
// => ''