---
title: Output Formats
description: Pick a built-in output format for ConsoleLogger, or write your own.
type: package
package: "@zap-studio/logger"
---

`ConsoleLogger` accepts a `format?: LogFormatter` option, defaulting to `classicFormat` — today's `message` + `context` output. Built-in formatters are all exported from `logger` (and the dedicated `@zap-studio/logger/format` subpath).

```ts
import { ConsoleLogger, jsonFormat } from "@zap-studio/logger";

const logger = new ConsoleLogger({ format: jsonFormat });
```

## classicFormat (default)

The message, plus `context` as a second `console` argument when present. Leaves object inspection to the runtime console — collapsible in browser devtools, `util.inspect` in Node.

```ts
logger.info("server started", { port: 3000 });
// console.info("server started", { port: 3000 })
```

## jsonFormat

Single-line JSON, pino-compatible field names (`time` as epoch milliseconds, `level`, `msg`):

```ts
logger.info("server started", { port: 3000 });
// {"port":3000,"time":1704067200000,"level":"info","msg":"server started"}
```

## compactFormat

Single-line `key=value` pairs (logfmt) — grep-friendly and still machine-parseable. Values containing whitespace, `"`, or `=` are quoted:

```ts
logger.info("server started", { port: 3000 });
// port=3000 time=2024-01-01T00:00:00.000Z level=info msg="server started"
```

## prettyFormat

Colorized, human-friendly single-line output: a dim local clock time, a color-coded level label, then the message. `context`, when present, is still passed as a second argument for native object inspection instead of being hand-formatted:

```ts
logger.info("server started", { port: 3000 });
// 12:34:56.789 INFO  server started   { port: 3000 }   (colored)
```

Color is applied automatically per runtime, with no configuration needed — see [Runtime Compatibility](/logger/runtime-compatibility) for exactly how it adapts on Node, Bun, Deno, browsers, and Cloudflare Workers.

## Field Merging (jsonFormat / compactFormat)

`context` fields are flattened to the top level rather than nested under a `context` key, matching the pino/bunyan convention:

```ts
logger.info("server started", { port: 3000 });
// jsonFormat: {"port":3000,"time":...,"level":"info","msg":"server started"}
```

A context field named `time`, `level`, or `msg` can never override the base field — the base fields are always applied last.

`Error` and `bigint` context values are handled specially, since neither survives `JSON.stringify` on its own (`JSON.stringify(new Error("boom"))` is `"{}"`):

```ts
logger.error("request failed", { error: new Error("boom") });
// jsonFormat:    ...,"error":{"name":"Error","message":"boom","stack":"..."}
// compactFormat: ... error="Error: boom"
```

## Writing Your Own

Any function matching `LogFormatter` works — no registration required:

```ts
import type { LogFormatter, LogRecord } from "@zap-studio/logger";

const upperFormat: LogFormatter = (record: LogRecord) => [record.message.toUpperCase()];

const logger = new ConsoleLogger({ format: upperFormat });
```

A `LogRecord` has `level`, `message`, `context` (`Record<string, unknown> | undefined`), and `timestamp` (a `Date`, built fresh for each call that passes the `minLevel` filter).

A `LogFormatter` returns the argument list `console[method](...)` gets called with — return one string for a single line, or more arguments (like `classicFormat` and `prettyFormat` do) to hand `context` off for native inspection.