If you are writing ES6 syntax in designer's script library, you will get an error - ""const is a reserve identified".

I think designer default to ES5 syntax checking.

To work around, you can write your javascript at "WebContent" folder using Package Explorer. For this example, I will put my javascript in "javascripts" folder.

This is how you point the javascript:

  <script src"javascript/like_button.js"></script>

Reference:
https://hclpnpsupport.hcltech.com/csm?id=community_question&sys_id=3bc76f951bbc481877761fc58d4bcb5e

Object Methods in Javascript

I have introduced "Merging Object via Object.assign()" in my previous article. In this article, I will go over other important build-in object methods.

Object.create()
Object.create() method is used to create a new object and link it to the prototype of an existing object.

Object.keys()
Object.keys() create an array containing the keys of an object.

Object.values()
Object.values() create an array containing the values of an object.

Object.entries()
Object.entries() creates a nested array of the key/value pairs of an object.

Object.assign() - I would like to mention again. :-)
Object.assign() is used to copy values from one object to another.

Object.freeze()
Object.freeze() prevents modification to properties and values of an object and prevents the property from being added or removed from an object.

Object.seal()
Object.seal() prevent new properties from being added to an object but allows the modification of existing properties.

Object.getPrototypeOf()
Object.getPrototypeOf() is used to get the internal hidden [[Prototype]] of an object, also accessible through the _proto_ property.

In this example, we can create an array and access to the Array prototype.

  const employees = ['Simon','Adam','John'];
  console.log(Object.getPrototypeOf(employees));

Output:

The output is showing the employee array has access to pop, find, and other array prototype methods.

This method can be useful to get information on the available prototype methods - String, Integer, and etc.

References: 
https://www.digitalocean.com/community/tutorials/how-to-use-object-methods-in-javascript