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2026-05-18-211-template-engine-dry.md

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Spec: Consolidate Template and Date Replacement Logic (DRY) — #211

Goal

Extract duplicated ${variable} template resolution into a single TemplateEngine service so that adding or changing a template variable requires editing exactly one place.

Why now

feat-210 (rich domain models) is in progress. Once its types land on develop, a TemplateEngine becomes a natural next refactoring step — the timing avoids double-churn on src/model/.

In scope

  • Extract replaceDateFormats and replaceDateTemplatesWithMomentsFormats from src/util/dates.ts into a new TemplateEngine module.
  • Replace the two parallel switch/case blocks with a single central variable map (TEMPLATE_VARIABLE_MAP) that holds both the moment format string and resolution logic per variable.
  • Expose two public functions from the engine:
    • resolveDate(template, date, locale?) — replaces ${var} with actual formatted values (replaces replaceDateFormats)
    • toMomentFormat(template) — replaces ${var} with moment format tokens (replaces replaceDateTemplatesWithMomentsFormats)
  • Delete the duplicate implementation in src/test/direct/path-parse-with-date.ts:124.
  • Update all callers: src/journal/paths.ts, src/vscode/conf.ts, src/features/sync/sync-daily-links.ts.
  • Keep replaceVariableValue in src/util/strings.ts (generic key/value; not template-engine-specific).
  • Update barrel exports in src/util/index.ts.

Out of scope

  • Changing the set of supported template variables (no new variables in this PR).
  • Replacing replaceVariableValue with the engine.
  • Any changes to src/vscode/conf.ts beyond updating the import path.
  • feat-210 rich domain model changes.

Acceptance criteria

  1. replaceDateFormats and replaceDateTemplatesWithMomentsFormats no longer exist in src/util/dates.ts.
  2. Adding a new template variable requires editing only TEMPLATE_VARIABLE_MAP (one location).
  3. All existing tests pass without modification to test assertions.
  4. The duplicate in src/test/direct/path-parse-with-date.ts is removed; that file imports from the engine instead.
  5. npm run compile (or equivalent) succeeds.
  6. No behavior change — output of resolveDate matches the old replaceDateFormats output for all supported variables.

Entities / contracts

TEMPLATE_VARIABLE_MAP

// Central map — the single source of truth
interface TemplateVariableEntry {
    momentFormat: string;  // used by toMomentFormat
}
const TEMPLATE_VARIABLE_MAP: Record<string, TemplateVariableEntry> = {
    year:      { momentFormat: 'YYYY' },
    month:     { momentFormat: 'MM' },
    day:       { momentFormat: 'DD' },
    localTime: { momentFormat: 'LT' },
    localDate: { momentFormat: 'LL' },
    weekday:   { momentFormat: 'dddd' },
    week:      { momentFormat: 'w' },    // only in resolveDate currently; add to toMomentFormat for symmetry
};

Public API (new module)

export function resolveDate(template: string, date: Date, locale?: string): string
export function toMomentFormat(template: string): string
// regex stays module-private
const TEMPLATE_VAR_REGEX = /\$\{(?:(year|month|day|localTime|localDate|weekday|week)|(d:[\s\S]+?))\}/g;

Module location — OPEN QUESTION

Issue suggests src/core/templates/engine.ts (new core layer). No src/core/ directory exists today. Alternative: src/util/template-engine.ts (fits existing structure; minimal disruption). Preferred: src/util/template-engine.ts unless the author wants to introduce the core layer for future domain-pure modules.

Constraints

  • Moment.js is the only date library in use; stay with it.
  • No new runtime dependencies.
  • Must not break src/test/direct/replace-variables-in-string.ts (imports replaceDateFormats from ../../util/dates — update import).

Open questions

  1. src/core/templates/engine.ts vs src/util/template-engine.ts? The core layer would be a new architectural boundary. Is that intentional for #211 or a future concern?
  2. week variable is present in replaceDateFormats but absent from replaceDateTemplatesWithMomentsFormats. Intentional gap or bug? Should toMomentFormat also map ${week} → 'w'?

Related issues

  • Blocks: none
  • Related: #210 (rich domain models — avoid double-churn; implement after feat-210 lands)
  • Related: #209 (domain module naming — completed; barrel keys already aligned)