README.md

Exercise 02 - Partial application: alert channel

The scenario

The Dino Safari control room is drowning in log noise. Every system - fences, feeders, motion sensors - pumps out messages, but operators need to see severity at a glance without repeating the severity label in every call site.

You'll use partial application to create lightweight, pre-configured alert functions. Each one "remembers" its severity so callers only have to supply the message. Then you'll layer on a second factory that tags alerts with a zone name, showing how partial application composes naturally.

What you will build

1. createAlertFn(severity) - in starter/create-alert-fn.js

Returns a function alert(message) that produces a formatted string:

[SEVERITY] message

For example:

const warn = createAlertFn('WARN');
warn('Fence voltage dropping'); // "[WARN] Fence voltage dropping"

const crit = createAlertFn('CRITICAL');
crit('Perimeter breach'); // "[CRITICAL] Perimeter breach"

• The severity is baked in at creation time - callers never pass it again.
• The returned function returns a string (no console.log needed).

2. createTaggedLogger(tag, baseAlertFn) - in starter/create-tagged-logger.js

Takes a zone tag and an existing alert function (like one produced by createAlertFn), and returns a new function log(message) whose output is:

TAG: baseAlertFn(message)

For example:

const crit = createAlertFn('CRITICAL');
const lagoon = createTaggedLogger('LAGOON', crit);
lagoon('breach'); // "LAGOON: [CRITICAL] breach"

This is partial application stacked two layers deep - the tag and the base function are both captured in the closure.

Getting started

Open the two stub files in starter/. Each has the right signature and a placeholder return - fill in the body. Then run starter/index.js to see them in action:

node starter/index.js

Verify

cd starter && pnpm install && pnpm test

The tests check formatting for createAlertFn and the composed output of createTaggedLogger.

Hints

• The outer function receives config; the inner function receives the per-call data. That's the partial application pattern.
• Template literals make the formatting straightforward.

Reference solution

solution/create-alert-fn.js | solution/create-tagged-logger.js