feat(logger): support pretty JSON output

.changeset/chubby-ads-speak.md

@@ -0,0 +1,5 @@
+---
+"@jamx/logger": patch
+---
+
+Add support for pretty JSON formatting

.github/workflows/check.yaml

@@ -24,7 +24,7 @@ jobs:
         run: pnpm install --frozen-lockfile
 
       - name: Install Playwright browsers
-        run: pnpx playwright install --with-deps
+        run: pnpm --filter @jamx/http exec playwright install --with-deps chromium
 
       - name: Run build
         run: pnpm build

packages/logger/README.md

@@ -74,6 +74,9 @@ logger.log(Severity.Info, "charge succeeded", {
 });
 ```
 
+`JsonFormatter` emits compact JSON by default. Pass an `indent` number or
+string for readable output, for example `new JsonFormatter({ indent: 2 })`.
+
 ## Convenience Logging API
 
 `CoreLogger` exposes the low-level `log(severity, message, meta)` method.
@@ -169,7 +172,7 @@ Built-in transports:
 Built-in formatters:
 
 - `PrettyFormatter`: human-readable console output.
-- `JsonFormatter`: JSON lines for structured output.
+- `JsonFormatter`: structured JSON output, with optional indentation.
 - `TextFormatter`: compact text output.
 - `PrintfFormatter`: printf-style message interpolation with metadata values.
 
@@ -235,6 +238,7 @@ Run examples from this package directory:
 ```bash
 pnpm run example:basic
 pnpm run example:structured
+pnpm run example:pretty-json
 pnpm run example:writer-pretty
 pnpm run example:context
 pnpm run example:processor
@@ -250,6 +254,7 @@ The example files live in `example/`:
 
 - `basic.ts`: named console logger with pretty output.
 - `structured.ts`: JSON output and structured metadata.
+- `pretty-json.ts`: indented JSON output with nested metadata.
 - `writer-pretty.ts`: pretty output written to stderr with `WriterTransport`.
 - `context.ts`: inherited metadata with child loggers.
 - `processor.ts`: redaction and enrichment with a processor.

packages/logger/example/pretty-json.ts

@@ -0,0 +1,21 @@
+import {
+  ConsoleTransport,
+  createNamedLogger,
+  JsonFormatter,
+  Severity,
+} from "../src/index.js";
+
+const logger = createNamedLogger({
+  name: "api",
+  minSeverity: Severity.Debug,
+  transport: new ConsoleTransport(new JsonFormatter({ indent: 2 })),
+});
+
+logger.info("Request completed", {
+  requestId: "req_123",
+  durationMs: 18,
+  response: {
+    status: 200,
+    cached: false,
+  },
+});

packages/logger/package.json

@@ -54,6 +54,7 @@
     "dev": "tsx ./example/basic.ts",
     "example:basic": "tsx ./example/basic.ts",
     "example:structured": "tsx ./example/structured.ts",
+    "example:pretty-json": "tsx ./example/pretty-json.ts",
     "example:writer-pretty": "tsx ./example/writer-pretty.ts",
     "example:context": "tsx ./example/context.ts",
     "example:processor": "tsx ./example/processor.ts",

packages/logger/src/internal/formatters/json.ts

@@ -1,13 +1,22 @@
 import { Formatter, LogRecord } from "../core.js";
 import { safeStringify } from "../shared.js";
 
+export interface JsonFormatterOptions {
+  indent?: number | string;
+}
+
 export class JsonFormatter implements Formatter {
+  constructor(private readonly options: JsonFormatterOptions = {}) {}
+
   format(log: LogRecord): string {
-    return safeStringify({
-      timestamp: log.timestamp.toISOString(),
-      severity: log.severityName,
-      message: log.message,
-      meta: log.meta,
-    });
+    return safeStringify(
+      {
+        timestamp: log.timestamp.toISOString(),
+        severity: log.severityName,
+        message: log.message,
+        meta: log.meta,
+      },
+      this.options.indent,
+    );
   }
 }

packages/logger/src/internal/shared.ts

@@ -23,30 +23,37 @@ export function assertMeta(
   }
 }
 
-export function safeStringify(value: unknown): string {
+export function safeStringify(
+  value: unknown,
+  space?: string | number,
+): string {
   const seen = new WeakSet<object>();
 
-  return JSON.stringify(value, (_, nestedValue: unknown) => {
-    if (nestedValue instanceof Error) {
-      return {
-        name: nestedValue.name,
-        message: nestedValue.message,
-        stack: nestedValue.stack,
-      };
-    }
-
-    if (typeof nestedValue === "bigint") {
-      return nestedValue.toString();
-    }
-
-    if (nestedValue && typeof nestedValue === "object") {
-      if (seen.has(nestedValue)) {
-        return "[Circular]";
+  return JSON.stringify(
+    value,
+    (_, nestedValue: unknown) => {
+      if (nestedValue instanceof Error) {
+        return {
+          name: nestedValue.name,
+          message: nestedValue.message,
+          stack: nestedValue.stack,
+        };
+      }
+
+      if (typeof nestedValue === "bigint") {
+        return nestedValue.toString();
       }
 
-      seen.add(nestedValue);
-    }
+      if (nestedValue && typeof nestedValue === "object") {
+        if (seen.has(nestedValue)) {
+          return "[Circular]";
+        }
+
+        seen.add(nestedValue);
+      }
 
-    return nestedValue;
-  });
+      return nestedValue;
+    },
+    space,
+  );
 }

packages/logger/test/index.test.ts

@@ -7,6 +7,7 @@ import {
   createNamedLogger,
   CompositeTransport,
   ConsoleTransport,
+  JsonFormatter,
   MemoryTransport,
   PrettyFormatter,
   PrintfFormatter,
@@ -316,3 +317,58 @@ describe("PrintfFormatter", () => {
     expect(record.message).toBe("hello %s");
   });
 });
+
+describe("JsonFormatter", () => {
+  it("formats compact JSON by default", () => {
+    const formatter = new JsonFormatter();
+
+    const output = formatter.format({
+      message: "Ready",
+      severityName: "debug",
+      severity: Severity.Debug,
+      timestamp: new Date("2026-03-03T11:12:35.123Z"),
+      meta: {},
+    });
+
+    expect(output).not.toContain("\n");
+  });
+
+  it("formats structured logs as indented JSON", () => {
+    const formatter = new JsonFormatter({ indent: 2 });
+
+    const output = formatter.format({
+      message: "Request completed",
+      severityName: "info",
+      severity: Severity.Info,
+      timestamp: new Date("2026-03-03T11:12:35.123Z"),
+      meta: {
+        requestId: "req_91f5",
+        durationMs: 18,
+      },
+    });
+
+    expect(output).toBe(`{
+  "timestamp": "2026-03-03T11:12:35.123Z",
+  "severity": "info",
+  "message": "Request completed",
+  "meta": {
+    "requestId": "req_91f5",
+    "durationMs": 18
+  }
+}`);
+  });
+
+  it("supports a custom indentation", () => {
+    const formatter = new JsonFormatter({ indent: "\t" });
+
+    const output = formatter.format({
+      message: "Ready",
+      severityName: "debug",
+      severity: Severity.Debug,
+      timestamp: new Date("2026-03-03T11:12:35.123Z"),
+      meta: {},
+    });
+
+    expect(output).toContain('\n\t"timestamp"');
+  });
+});