prefer-assertions-over-defensive-checks

Prefer Assertions Over Defensive Checks

CRITICAL: When data has already been validated, use assertions instead of defensive checks. This eliminates unrealistic test scenarios and makes invalid states unrepresentable.

The Problem

Defensive checks with optional chaining create code paths that are impossible in practice — and force tests that bypass TypeScript to exercise impossible states (coverage padding), break when types are tightened, and add maintenance burden.

// ❌ WRONG: Defensive check after validation
if (!contractTable.columns[columnName]) {
  errorUnknownColumn(columnName, tableName);
}
const columnMeta = contractTable.columns[columnName];
const codecId = columnMeta?.codecId; // Optional chaining unnecessary
if (codecId && paramName) paramCodecs[paramName] = codecId;
paramDescriptors.push({
  ...(codecId ? { type: codecId } : {}),
  ...(columnMeta?.nullable !== undefined ? { nullable: columnMeta.nullable } : {}),
});

// ✅ CORRECT: Use assertions after validation
if (!contractTable.columns[columnName]) {
  errorUnknownColumn(columnName, tableName);
}
const columnMeta = contractTable.columns[columnName]!; // assert existence
const codecId = columnMeta.codecId;                    // required property, no optional chaining
if (paramName) paramCodecs[paramName] = codecId;
paramDescriptors.push({ type: codecId, nullable: columnMeta.nullable }); // required, no optional check

When to use assertions

• Data has been validated: you've already checked it exists/is valid (e.g. after if (!contractTable.columns[columnName]) use columnMeta!; after if (!model) use model!).
• Type system guarantees it: TypeScript types require the property.
• Contract validation ensures it: runtime validation (e.g. validateContract) guarantees the structure, so access required properties directly.

When NOT to use assertions

• Data is truly optional: the property might legitimately be missing.
• External input: data from user input, APIs, or files that hasn't been validated.
• Unvalidated contracts: contract data that hasn't been validated yet.