TypeScript Lab: Type Narrowing Switchboard

LAB / EXPLORATION

TypeScript Lab: Type Narrowing Switchboard

Planned

LAB 019 — TypeScript Data Modeling
Reveal fields only after a union value is narrowed.

LAB TypeScript Lab: Type Narrowing Switchboard Preview coming soon

LAB 019

TypeScript Lab: Type Narrowing Switchboard

Concept

Narrowing proves which variant is active before variant-specific properties are used.

Concept

Narrowing proves which variant is active before variant-specific properties are used.

Objective & Explanation

What the project should make visible and why the concept matters.

Learning Target

Objective

Reveal fields only after a union value is narrowed.

Explanation

Narrowing proves which variant is active before variant-specific properties are used.

Project Plan

Structure, technologies, data boundary, and source connection.

Structure

project/
├── package.json
├── tsconfig.json
├── index.html
├── styles.css
└── src/
    └── app.ts

Technologies

TypeScript
HTML
CSS
JSON

Data Layer

Use a small local object, array, or JSON fixture so the transformation stays visible.

Sandbox Source

src/datastore/types.ts

Build Instructions

Build only the focused behavior, then use the visual layer to inspect it.

Setup

Create the standalone project folder and listed files.

Implement

Implement the smallest version that demonstrates the concept.

Run & Inspect

Change one input and observe the visible output change.

Code & Configuration

Starter configuration plus the focused implementation example.

Focused TypeScript

Implement this behavior first, then alter the input.

type Result =
  | { kind: "success"; data: string }
  | { kind: "error"; message: string };

function label(result: Result): string {
  return result.kind === "success" ? result.data : result.message;
}

Starter Configuration

Keep the project independently buildable.

package.json

{
  "type": "module",
  "scripts": { "build": "tsc" },
  "devDependencies": { "typescript": "^7.0.2" }
}

tsconfig.json

{
  "compilerOptions": {
    "rootDir": "./src",
    "outDir": "./dist",
    "target": "ES2022",
    "module": "ES2022",
    "lib": ["ES2022", "DOM"],
    "strict": true,
    "sourceMap": true
  },
  "include": ["src/**/*.ts"]
}

Expected Visual Result

The output is the proof that the concept is working.

Visible Proof

The browser should visibly demonstrate: reveal fields only after a union value is narrowed.

Data: JSON