Code is written once and read many times. Your code will live far longer than you probably think, and some poor soul will need to change it long after you’ve moved on.

While this should elevate the importance of genuinely useful comments, I rarely see comments which explain why code exists, what else was tried (that failed), and after minutes or hours of research, what specifically convinced the dev that this particular implementation was worth using.

Fake example:

// We sync the history with local storage at this point because it's already
// filtered down to items which need to be persisted across page loads.

I feel like this topic is programming 101 – good comments explain why! – but even “good” developers consistently fail to do it.

And now a real example:

# CartoDB installations assume UTF-8.
# shows that localedef is already run when using the official Postgres image,
# but we must RUN IT AGAIN for some reason... initdb will otherwise fail.
RUN localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
ENV LANG en_US.utf8

Without the comment, some clever dev might think “This is wrong. Debian Jessie sets the localedef and here it’s set again! This is safe to remove.” But you know what? I spent over an hour tracking down localedef issues in this container. I am absolutely sure that “cleaning up” those lines will break the environment.

Another example:

CREATE TABLE "public"."sys_organizations" (
  "organization_id" UUID NOT NULL,
  "name" VARCHAR(64) NOT NULL,
  -- We want the slug length limit = 20, but citext type does not
  -- support length modifiers.
  -- A RegEx constraint is used instead, see ck_organization_slug_must_match_regex.
  "created_date" TIMESTAMP NOT NULL,
  "last_modified_date" TIMESTAMP,
CONSTRAINT "pk_sys_organizations" PRIMARY KEY ("organization_id")

Again without comments, some hotshot (probably me, 8 months down the line) will think “The DB docs say that slugs have a max length 20. If I constrain the length here, then our column will enforce the rule.” …and they would be wrong. It’s not common knowledge that citext doesn’t support length modifiers. Sure, they’d find out after a quick Google search, but how long will it take to figure out that we’ve used a RegEx constraint instead? Too long.

Final example:

// --------------------------------------------------------------------------
// -- This gateway is for members to get and change info within their org. --
// -- The caller MUST assert org-level permissions. These functions and    --
// -- stored procs filter data by the member's org, but they do not check  --
// -- the member's roles or permissions.                                   --
// --------------------------------------------------------------------------

This type of comment I consider most important. For anyone new to your codebase, security decisions are most critical to understand. If your decisions are locked away into some “Security Architecture of Project ABC” PDF file, then you’ve failed. Make your security critical code read like a book; it sets up the maintainers to win.