3.3.1-alpha-4 • Public • Published


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This module provides a basic ndjson formatter. If an incoming line looks like it could be a log line from an ndjson logger, in particular the Pino logging library, then it will apply extra formatting by considering things like the log level and timestamp.

A standard Pino log line like:

{"level":30,"time":1522431328992,"msg":"hello world","pid":42,"hostname":"foo","v":1}

Will format to:

[1522431328992] INFO (42 on foo): hello world


Using the example script from the Pino module, and specifying that logs should be colored and the time translated, we can see what the prettified logs will look like:



$ npm install -g pino-pretty


It's recommended to use pino-pretty with pino by piping output to the CLI tool:

pino app.js | pino-pretty

CLI Arguments

  • --colorize (-c): Adds terminal color escape sequences to the output.
  • --crlf (-f): Appends carriage return and line feed, instead of just a line feed, to the formatted log line.
  • --errorProps (-e): When formatting an error object, display this list of properties. The list should be a comma separated list of properties Default: ''.
  • --levelFirst (-l): Display the log level name before the logged date and time.
  • --errorLikeObjectKeys (-k): Define the log keys that are associated with error like objects. Default: err,error.
  • --messageKey (-m): Define the key that contains the main log message. Default: msg.
  • --timestampKey (-m): Define the key that contains the log timestamp. Default: time.
  • --translateTime (-t): Translate the epoch time value into a human readable date and time string. This flag also can set the format string to apply when translating the date to human readable format. For a list of available pattern letters see the dateformat documentation.
    • The default format is yyyy-mm-dd HH:MM:ss.l o in UTC.
    • Require a SYS: prefix to translate time to the local system's timezone. A shortcut SYS:standard to translate time to yyyy-mm-dd HH:MM:ss.l o in system timezone.
  • --search (-s): Specify a search pattern according to jmespath.
  • --ignore (-i): Ignore one or several keys: (-i time,hostname)

Programmatic Integration

We recommend against using pino-pretty in production, and highly recommend installing pino-pretty as a development dependency.

When installed, pino-pretty will be used by pino as the default prettifier.

Install pino-pretty alongside pino and set the prettyPrint option to true:

const pino = require('pino')
const logger = pino({
  prettyPrint: true


The prettyPrint option can also be an object containing pretty-print options:

const pino = require('pino')
const logger = pino({
  prettyPrint: { colorize: true }


See the Options section for all possible options.


pino-pretty exports a factory function that can be used to format log strings. This factory function is used internally by Pino, and accepts an options argument with keys corresponding to the options described in CLI Arguments:

  colorize: chalk.supportsColor, // --colorize
  crlf: false, // --crlf
  errorLikeObjectKeys: ['err', 'error'], // --errorLikeObjectKeys
  errorProps: '', // --errorProps
  levelFirst: false, // --levelFirst
  messageKey: 'msg', // --messageKey
  timestampKey: 'time', // --timestampKey
  translateTime: false, // --translateTime
  search: 'foo == `bar`', // --search
  ignore: 'pid,hostname', // --ignore
  processors: undefined

The colorize default follows chalk.supportsColor.

Note: the logParsers and lineBuilders options do not correspond to CLI arguments. They are available only for API usage, as described below.

log processors

A log processor is an object that parses a log entry and builds the corresponding formatted line sent to the output stream. The log processing sequence is composed of a series of log processors, each of which handles a particular property or aspect of the input.

A log processor can be a parser, a builder, or both. Accordingly, the log processing sequence has two stages: a parsing stage and a building stage. All parsers are executed in order, and then all builders are executed in the same order. The output of the parsers is passed as input to the builders.

A parser parses an entry and can change it before it is passed to subsequent parsers. It can also modify the context object, which makes it possible to pass data between parsers, or between the parse and build stages of a log processor that supports both stages.

pino-pretty uses the following built-in log processors, in this order:

  • json (parser) - Parses the input ndjson string as an object, which is subsequently passed to the remaining log processors.
  • primitives (parser) - If the parsed result of the json log processor is null, a boolean value, or a number, the log processing sequence for the current line is aborted (short-circuited) and the value is returned.
  • search (parser) - If the search property is specified, filters out entries that do not match the search value, based on the jmespath library.
  • ignore (parser) - Removes the specified properties from the input object.
  • time (parser, builder) - Prettifies a timestamp if the given input object has either time, timestamp or custom specified timestamp property.
  • level (parser, builder) - Checks if the passed in input object has a level value and returns a prettified string for that level if so.
  • metadata (parser, builder) - Prettifies metadata that is usually present in a Pino log line. It looks for fields name, pid, and hostname and returns a formatted string using the fields it finds.
  • semicolon (builder) - Appends a semicolon to the output if values have already been added to the output.
  • message (parser, builder) - Prettifies a message string if the given input object has a message property.
  • eol (builder) - Adds an end-of-line to the output. Uses \r\n if the crlf option is true or \n otherwise. Defaults to \n.
  • error (builder) - Given a log object that has a type: 'Error' key, prettifies the object and returns the result.
  • object (builder) - Prettifies a standard object. Special care is taken when processing the object to handle child objects that are attached to keys known to contain error objects.

The built-in log processing sequence can be replaced with a custom sequence comprised of a mix of built-in and custom log processors. To customize the log processing sequence, pass an array of log processors to the processors option. They will be executed in the order in which they are specified.

To use a built-in log processor in the custom sequence, simply add a string specifying its name to the processors array. Any of the log processors in the list above can be used, except for the json log processor.

Do not add the built-in json log processor to the custom sequence. The json log processor is always executed first, even when the processors option is specified. It cannot be removed or replaced. This is intended to ensure that any subsequent log processors in the sequence will always operate on an object consistently parsed from the original ndjson input.

The default sequence is equivalent to specifying the processors option as follows:

  processors: [
    // 'json', - the `json` log processor is always implicitly added at the beginning

By changing the order of the built-in log processors or excluding some of them, the output can changed. For example, to only show the time, level and actual message, while reversing the order of the time and level, the sequence can be specified as follows: { processors: ['level', 'time', 'message'] }.

Custom log processors can also be added to the log processing sequence. To add a custom log processor, use one of the following structures:

  • parser object
      parse(input, context) {}
  • builder object
      build(lineParts, context) {}
  • complete parser and builder object
      parse(input, context) {}
      build(lineParts, context) {}
  • parser function
    (input, context) => {}

Only declare the parse or build method that is needed. Declaring a method that does nothing will negatively affect performance.


The parse function receives three parameters:input, context, and state.

  • The input object represents the current log entry, possibly modified by the previously-executed parsers.

  • The context object represents the options, settings, and other data used by the log processors. The options object used to initialize Pino, is merged into the context object, making those settings available to overy log processor.

    The following properties are always available as well:

    • EOL: the actual end-of-line characters, as specified by the crlf option
    • IDENT: the default indentation string
    • translateFormat: the time formatting string, as specified by the translateTime option
    • colorizer: the selected colorizer function that accepts a level value and returns a colorized string
    • prettified: an object that caches prettified text, whose properties are typically set by a parser and consumed by a builder
  • The state object has a stop method, which is used to abort the log processing process for the current entry immediately after the current parser returns.

The return value of the parse function is passed to the next parse function in the sequence. The value returned from the last parse function is passed to the build function.

For most parsers, the return value should be a log entry object that will be used by the builders to construct the output line.

In some cases, whether for performance or other reasons, it makes sense to return the result of a parser directly without performing any additional processing. The process can be short-circuited by calling the stop method of the state argument. No other parsers will be used, and the build stage will be skipped as well. The value returned by the parser that calls the stop function will be returned as the final formatted output.


The build function is a simple functions that prepares the final formatted line. It receives two parameters: lineParts and context.

  • The lineParts array contains the ordered list of strings that will eventually be joined after all the build functions have been executed.

  • The context object is the same context object passed to the parsers, with one exception: the final output produced by the parsers is added to the object as the log property.

    Also, when the built-in prettifying log processors are used, a number of prettified strings are added to the context.prettified property: prettifiedLevel, prettifiedMessage, prettifiedMetadata, and prettifiedTime.

The builder function does not return a value. To change the output, modify the lineParts array.

Note that if a parser short-circuits the parsing process, the builders will not be executed at all.


The following are a number of examples of parsers, builders, and combined log processors:

  • parser - Perform a transformation on the input message:

    function (input) {
      input.msg = input.msg.toLowerCase()
      return input
  • parser - Short-circuit the parsing process and prevent subsequent log parsers from being executed:

    (input, context, state) => {
      state.stop() // short-circuit the log parsing process
      return input
  • parser - Prepare a prettified string:

      parse(input, { colorizer, inlineSection ) {
        if(inlineSection && 'section' in input) {
          context.prettified.section = `[${colorizer(input.section)}]`
        return input
  • builder - Add a value to the array:

    function (lineParts) {
      lineParts.push('NEW VALUE')
  • builder - The following is an example of a more complex builder function, based on one of the built-in builders:

      build (lineParts, { prettified }) => {
        const { prettifiedTime } = prettified
        if (prettifiedTime) {
          if (lineParts.length > 0) {
            lineParts.push(' ')
  • parser and builder - Parse a log entry and then use the result to build the output line:

      parse(input, { colorizer, inlineSection ) {
        if(inlineSection && 'section' in input) {
          context.prettified.section = ${colorizer(input.section)}
        return input
      build (lineParts, { prettified }) => {
        const { section } = prettified
        if (section) {
          if (lineParts.length > 0) {
            lineParts.push(' ')


MIT License



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