Union types

Problem

What becomes confusing, fragile, or impossible without understanding union types? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

TypeScript added gradual static analysis to JavaScript without replacing its runtime, preserving ecosystem compatibility through erasure.

Concept explanation

Union types belongs to “Inference and narrowing”. Type inference and narrowing reduce possible runtime states but external values begin as unknown. This lesson turns union types into a working part of weekly fixture.

For union types, trace concrete input, state transition, output, and failure through a compile-time relationship that constrains JavaScript while leaving runtime values and external data untrusted.

Types constrain possible programs during analysis, then disappear. Runtime values remain JavaScript values and external data remains untrusted. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.ts. Focus: Build union types around this exact fixture: Model loading/success/error union and exhaustively render each state.

1. Run: npx tsc --strict --noEmit lab.ts
2. Save baseline evidence. Read inferred types, diagnostics, emitted JavaScript, declarations, module traces, and source maps.
3. Boundary case: Add union types support for weekly boundary case: Parse unknown JSON through runtime guard.
4. Failure case: Make union types surface—not hide—this failure: Add new union member and require compiler to expose missing branch.

RESULT
union types handles baseline and boundary while preserving this observable result: Strict compile passes only with exhaustive handling and runtime validation at boundary. Starter-level baseline: Strict type checking exits successfully. Changing result.value to a string-only operation produces a compiler diagnostic.

Starter material

Create lab.ts, paste this exact content, then run the command below.

type Observation<T> = { topic: string; value: T; valid: boolean };

function observe<T>(topic: string, value: T): Observation<T> {
  return { topic, value, valid: value !== undefined };
}

const result = observe("union types", 1);
console.log(result.value);

STOP / CLEANUPCompiler exits by itself; no cleanup command required.

Independent exercise

Add one real case: union types

1. Create second case from blank file: Add union types support for weekly boundary case: Parse unknown JSON through runtime guard.
2. Then create controlled failure: Make union types surface—not hide—this failure: Add new union member and require compiler to expose missing branch.
3. Use tsc · hx LSP · emitted JS · source maps to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• union types handles baseline and boundary while preserving this observable result: Strict compile passes only with exhaustive handling and runtime validation at boundary.
• Strict type checking exits successfully. Changing result.value to a string-only operation produces a compiler diagnostic.
• Controlled union types failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.