Module resolution

Problem

What becomes confusing, fragile, or impossible without understanding module resolution? 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

Module resolution belongs to “The erased boundary”. TypeScript erases to JavaScript; configuration, modules, declarations, and source maps define build boundary. This lesson turns module resolution into a working part of weekly fixture.

For module resolution, 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 module resolution around this exact fixture: Compile one module to JS and declarations, then inspect both.

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 module resolution support for weekly boundary case: Trace resolution under two module settings.
4. Failure case: Make module resolution surface—not hide—this failure: Pass invalid network JSON into asserted type, observe runtime bug, add validator.

RESULT
module resolution handles baseline and boundary while preserving this observable result: Emitted runtime contains no types; resolution trace is explained; invalid external data is rejected. 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("module resolution", 1);
console.log(result.value);

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

Independent exercise

Add one real case: module resolution

1. Create second case from blank file: Add module resolution support for weekly boundary case: Trace resolution under two module settings.
2. Then create controlled failure: Make module resolution surface—not hide—this failure: Pass invalid network JSON into asserted type, observe runtime bug, add validator.
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

• module resolution handles baseline and boundary while preserving this observable result: Emitted runtime contains no types; resolution trace is explained; invalid external data is rejected.
• Strict type checking exits successfully. Changing result.value to a string-only operation produces a compiler diagnostic.
• Controlled module resolution failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.