This Array-method is a fairly new addition to the ECMAScript specification, hence we need a polyfill to avoid the library/viewer breaking in older browsers. Please find additional information at: - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at#browser_compatibility