Add Make Remove Column Migration Reversible as a Rails TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1225 TILs and counting..._
+_1227 TILs and counting..._
 
 ---
 
@@ -408,6 +408,7 @@ _1225 TILs and counting..._
 - [for...in Iterates Over Object Properties](javascript/for-in-iterates-over-object-properties.md)
 - [Formatting Values With Units For Display](javascript/formatting-values-with-units-for-display.md)
 - [Freeze An Object, Sorta](javascript/freeze-an-object-sorta.md)
+- [Generate A V4 UUID In The Browser](javascript/generate-a-v4-uuid-in-the-browser.md)
 - [Generate Random Integers](javascript/generate-random-integers.md)
 - [Get The Location And Size Of An Element](javascript/get-the-location-and-size-of-an-element.md)
 - [Get The Response Status From An Axios Error](javascript/get-the-response-status-from-an-axios-error.md)
@@ -766,6 +767,7 @@ _1225 TILs and counting..._
 - [Log SQL Queries Executed By ActiveRecord](rails/log-sql-queries-executed-by-activerecord.md)
 - [Mark A Migration As Irreversible](rails/mark-a-migration-as-irreversible.md)
 - [Make ActionMailer Synchronous In Test](rails/make-action-mailer-synchronous-in-test.md)
+- [Make Remove Column Migration Reversible](rails/make-remove-column-migration-reversible.md)
 - [Manually Run A Migration From Rails Console](rails/manually-run-a-migration-from-rails-console.md)
 - [Mark For Destruction](rails/mark-for-destruction.md)
 - [Mask An ActiveRecord Attribute](rails/mask-an-activerecord-attribute.md)

javascript/generate-a-v4-uuid-in-the-browser.md

@@ -0,0 +1,24 @@
+# Generate A V4 UUID In The Browser
+
+The [Web Crypto
+API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Crypto_API) provides
+a
+[`randomUUID`](https://developer.mozilla.org/en-US/docs/Web/API/Crypto/randomUUID)
+function for generating v4 UUID values which are cryptographically random.
+
+It works like this:
+
+```javascript
+> crypto.randomUUID()
+'f2050c5e-af52-4ca2-b4d6-23758b3396c9'
+> crypto.randomUUID()
+'079d5186-84d4-41d6-a660-edafb6a74c86'
+```
+
+No two UUIDs will be the same. This is a great way to generate IDs that are
+guaranteed to be unique. Or if you need a value that is practically impossible
+to guess.
+
+I was surprised to see that as of writing this, this function has great browser
+support. Modern versions of all browsers except Internet Explorer have
+implemented `randomUUID`.

rails/make-remove-column-migration-reversible.md

@@ -0,0 +1,32 @@
+# Make Remove Column Migration Reversible
+
+The Rails migration DSL includes a
+[`#remove_column`](https://edgeapi.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-remove_column)
+method for removing a column from a table.
+
+The two required values are the table name and the column name.
+
+```ruby
+class RemoveNotesColumnFromTodo < ActiveRecord::Migration[5.2]
+  def change
+    remove_column :todos, :notes
+  end
+end
+```
+
+This on it's own is not reversible though. If you were to run a `db:rollback`
+with this migration, Rails wouldn't know what type to use when re-adding the
+column.
+
+To make it reversible, you need to add in the column type as a third argument.
+
+```ruby
+    remove_column :todos, :notes, :string
+```
+
+Note: perhaps more importantly, this column isn't reversible in the sense that
+when you remove the column, you are throwing away all the data associated with
+that column. Be sure you're ready to part with this data before deploying a
+migration like this.
+
+[source](https://stackoverflow.com/questions/24520550/how-do-you-make-remove-column-reversible)