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This Is a Commit Snapshot of the Standard

This document contains the contents of the standard as of the b394072ad8403ae75ce8d13bfbcca6bcac16cae4 commit, and should only be used as a historical reference. This commit may not even have been merged into master.

Do not attempt to implement this version of the specification. Do not reference this version as authoritative in any way. Instead, see https://console.spec.whatwg.org/ for the living standard.

Abstract

This specification defines APIs for console debugging facilities.

Status

This specification is an early work in progress that welcomes feedback to refine toward more precise and compatible definitions. It is also the editors' first specification, so please be kind and constructive.

Please join us in the issue tracker for more discussion.

1. Namespace console

[Exposed=(Window,Worker,Worklet)]
namespace console { // but see namespace object requirements below
  // Logging
  void assert(optional boolean condition = false, any... data);
  void clear();
  void count(optional DOMString label = "default");
  void debug(any... data);
  void error(any... data);
  void info(any... data);
  void log(any... data);
  void table(any tabularData, optional sequence<DOMString> properties);
  void trace(any... data);
  void warn(any... data);
  void dir(any item, optional object? options);
  void dirxml(any... data);

  // Grouping
  void group(any... data);
  void groupCollapsed(any... data);
  void groupEnd();

  // Timing
  void time(optional DOMString label = "default");
  void timeEnd(optional DOMString label = "default");
};

For historical reasons, console is lowercased.

It is important that console is always visible and usable to scripts, even if the developer console has not been opened or does not exist.

For historical web-compatibility reasons, the namespace object for console must have as its [[Prototype]] an empty object, created as if by ObjectCreate(%ObjectPrototype%), instead of %ObjectPrototype%.

1.1. Logging methods

Each console namespace object has an associated count map, which is a map of strings to labels, initially empty.

1.1.1. assert(condition, ...data)

  1. If condition is true, return.

  2. Let message be a string without any formatting specifiers indicating generically an assertion failure (such as "Assertion failed").

  3. If data is empty, append message to data.

  4. Otherwise:

    1. Let first be data[0].

    2. If Type(first) is not String, then prepend message to data.

    3. Otherwise:

      1. Let concat be the concatenation of message, U+003A (:), U+0020 SPACE, and first.

      2. Set data[0] to concat.

  5. Perform Logger("assert", data).

1.1.2. clear()

  1. Empty the appropriate group stack.

  2. If possible for the environment, clear the console. (Otherwise, do nothing.)

1.1.3. count(label)

  1. Let map be the associated count map.

  2. If map[label] exists, set map[label] to map[label] + 1.

  3. Otherwise, set map[label] to 1.

  4. Let concat be the concatenation of label, U+003A (:), U+0020 SPACE, and ToString(map[label]).

  5. Perform Logger("count", « concat »).

1.1.4. debug(...data)

  1. Perform Logger("debug", data).

1.1.5. error(...data)

  1. Perform Logger("error", data).

1.1.6. info(...data)

  1. Perform Logger("info", data).

1.1.7. log(...data)

  1. Perform Logger("log", data).

1.1.8. table(tabularData, properties)

Try to construct a table with the columns of the properties of tabularData (or use properties) and rows of tabularData and log it with a logLevel of "log". Fall back to just logging the argument if it can’t be parsed as tabular.

TODO: This will need a good algorithm.

1.1.9. trace(...data)

  1. Let trace be some implementation-specific, potentially-interactive representation of the callstack from where this method was called.

  2. Optionally, let formattedData be the result of Formatter(data), and incorporate formattedData as a label for trace.

  3. Perform Printer("trace", « trace »).

The identifier of a function printed in a stack trace is implementation-dependant. It is also not guaranteed to be the same identifier that would be seen in new Error().stack.

1.1.10. warn(...data)

  1. Perform Logger("warn", data).

1.1.11. dir(item, options)

  1. Let object be item with generic JavaScript object formatting applied.

  2. Perform Printer("dir", « object », options).

1.1.12. dirxml(...data)

  1. Let finalList be a new list, initially empty.

  2. For each item of data:

    1. Let converted be a DOM tree representation of item if possible; otherwise let converted be item with optimally useful formatting applied.

    2. Append converted to finalList.

  3. Perform Logger("dirxml", finalList).

1.2. Grouping methods

A group is an implementation-specific, potentially-interactive view for output produced by calls to Printer, with one further level of indentation than its parent. Each console namespace object has an associated group stack, which is a stack, initially empty. Only the last group in a group stack will host output produced by calls to Printer.

1.2.1. group(...data)

  1. Let group be a new group.

  2. If data is not empty, let groupLabel be the result of Formatter(data). Otherwise, let groupLabel be an implementation-chosen label representing a group.

  3. Incorporate groupLabel as a label for group.

  4. Optionally, if the environment supports interactive groups, group should be expanded by default.

  5. Perform Printer("group", « group »).

  6. Push group onto the appropriate group stack.

1.2.2. groupCollapsed(...data)

  1. Let group be a new group.

  2. If data is not empty, let groupLabel be the result of Formatter(data). Otherwise, let groupLabel be an implementation-chosen label representing a group.

  3. Incorporate groupLabel as a label for group.

  4. Optionally, if the environment supports interactive groups, group should be collapsed by default.

  5. Perform Printer("groupCollapsed", « group »).

  6. Push group onto the appropriate group stack.

1.2.3. groupEnd()

  1. Pop the last group from the group stack.

1.3. Timing methods

Each console namespace object has an associated timer table, which is a map of strings to times, initially empty.

1.3.1. time(label)

  1. If the associated timer table contains an entry with key label, return, optionally reporting a warning to the console indicating that a timer with label label has already been started.

  2. Otherwise, set the value of the entry with key label in the associated timer table to the current time.

1.3.2. timeEnd(label)

  1. Let startTime be the result of getting the value of the entry with key label in the associated timer table.

  2. Let duration be a string representing the difference between the current time and startTime, in an implementation-defined format.

    "4650", "4650.69 ms", "5 seconds", and "00:05" are all reasonable ways of displaying a 4650.69 ms duration.

  3. Let concat be the concatenation of label, U+003A (:), U+0020 SPACE, and duration.

  4. Perform Logger("timeEnd", « concat »).

2. Supporting abstract operations

2.1. Logger(logLevel, args)

The logger operation accepts a log level and a list of other arguments. Its main output is the implementation-defined side effect of printing the result to the console. This specification describes how it processes format specifiers while doing so.

  1. If args is empty, return.

  2. Let first be args[0].

  3. Let rest be all elements following first in args.

  4. If rest is empty, perform Printer(logLevel, « first ») and return.

  5. If first does not contain any format specifiers, perform Printer(logLevel, args).

  6. Otherwise, perform Printer(logLevel, Formatter(args)).

  7. Return undefined.

It’s important that the printing occurs before returning from the algorithm. Many developer consoles print the result of the last operation entered into them. In such consoles, when a developer enters console.log("hello!"), this will first print "hello!", then the undefined return value from the console.log call.

Indicating that printing is done before return

2.2. Formatter(args)

The formatter operation tries to format the first argument provided, using the other arguments. It will try to format the input until no formatting specifiers are left in the first argument, or no more arguments are left. It returns a list of objects suitable for printing.

  1. Let target be the first element of args.

  2. Let current be the second element of args.

  3. Find the first possible format specifier specifier, from the left to the right in target.

    1. If specifier is %s, let converted be the result of ToString(current).

    2. If specifier is %d or %i, let converted be the result of %parseInt%(current, 10).

    3. If specifier is %f, let converted be the result of %parseFloat%(current).

    4. If specifier is %o, optionally let converted be current with optimally useful formatting applied.

    5. If specifier is %O, optionally let converted be current with generic JavaScript object formatting applied.

    6. TODO: process %c

    7. If any of the previous steps set converted, replace specifier in target with converted.

    8. Let result be a list containing target together with the elements of args starting from the third onward.

  4. If target does not have any format specifiers left, return result.

  5. If result’s size is 1, return result.

  6. Return Formatter(result).

2.2.1. Summary of formatting specifiers

The following is an informative summary of the format specifiers processed by the above algorithm.

Specifier Purpose Type Conversion
%s Element which substitutes is converted to a string ToString(element)
%d or %i Element which substitutes is converted to an integer %parseInt%(element, 10)
%f Element which substitutes is converted to a float %parseFloat%(element, 10)
%o Element is displayed with optimally useful formatting n/a
%O Element is displayed with generic JavaScript object formatting n/a
%c Applies provided CSS n/a

2.3. Printer(logLevel, args[, options])

The printer operation is implementation-defined. It accepts a log level indicating severity, a List of arguments to print, and an optional object of implementation-specific formatting options. Elements appearing in args will be one of the following:

If the options object is passed, and is not undefined or null, implementations may use options to apply implementation-specific formatting to the elements in args.

How the implementation prints args is up to the implementation, but implementations should separate the objects by a space or something similar, as that has become a developer expectation.

By the time the printer operation is called, all format specifiers will have been taken into account, and any arguments that are meant to be consumed by format specifiers will not be present in args. The implementation’s job is simply to print the List. The output produced by calls to Printer should appear only within the last group on the appropriate group stack if the group stack is not empty, or elsewhere in the console otherwise.

If the console is not open when the printer operation is called, implementations should buffer messages to show them in the future up to an implementation-chosen limit (typically on the order of at least 100).

2.3.1. Indicating logLevel severity

Each console method uses a unique value for the logLevel parameter when calling Printer, allowing implementations to customize each printed message depending on the method from which it originated. However, it is common practice to group together certain methods and treat their output similarly, in four broad categories. This table summarizes these common groupings:

Grouping console methods Description
log log(), trace(), dir(), dirxml(), group(), groupCollapsed(), debug() A generic log
info count(), info(), timeEnd() An informative log
warn warn() A log warning the user of something indicated by the message
error error(), assert() A log indicating an error to the user

These groupings are meant to document common practices, and do not constrain implementations from providing special behavior for each method, as in the following examples:

Here you can see one implementation chose to make output produced by calls to timeEnd() blue, while leaving info() a more neutral color.

A demonstration of timeEnd and info formatting differences

Calls to count() might not always print new output, but instead could update previously-output counts.

A demonstration of count behavior

2.3.2. Printer user experience innovation

Since Printer is implementation-defined, it is common to see UX innovations in its implementations. The following is a non-exhaustive list of potential UX enhancements:

2.3.3. Common object formats

Typically objects will be printed in a format that is suitable for their context. This section describes common ways in which objects are formatted to be most useful in their context. It should be noted that the formatting described in this section is applied to implementation-specific object representations that will eventually be passed into Printer, where the actual side effect of formatting will be seen.

An object with generic JavaScript object formatting is a potentially expandable representation of a generic JavaScript object. An object with optimally useful formatting is an implementation-specific, potentially-interactive representation of an object judged to be maximally useful and informative.

2.3.4. Example printer in Node.js

The simplest way to implement the printer operation on the Node.js platform is to join the previously formatted arguments separated by a space and write the output to stdout or stderr.

Example implementation in Node.js using [ECMASCRIPT]:

const util = require('util');

function print(logLevel, ...args) {
  const message = util.format(...args);

  if (logLevel === 'error') {
    process.stderr.write(message + '\n');
  } else if (logLevel === 'log' || logLevel === 'info' || logLevel === 'warn') {
    process.stdout.write(message + '\n');
  }
}

Here a lot of the work is done by the util.format function. It stringifies nested objects, and converts non-string arguments into a readable string version, e.g. undefined becomes the string "undefined" and false becomes "false":

print('log', 'duck', [{foo: 'bar'}]);     // prints: duck [ { foo: 'bar' } ]\n on stdout
print('log', 'duck', false);              // prints: duck false\n on stdout
print('log', 'duck', undefined);          // prints: duck undefined\n on stdout

Acknowledgments

The editors would like to thank Boris Zbarsky, Brent S.A. Cowgill, Brian Grinstead, Corey Farwell, Ian Kilpatrick, Jeff Carpenter, Joseph Pecoraro, Justin Woo, Noah Bass, Paul Irish, Raphaël, and Victor Costan for their contributions to this specification. You are awesome!

This standard is written by Terin Stock (terin@terinstock.com), Robert Kowalski (rok@kowalski.gd), and Dominic Farolino (domfarolino@gmail.com) with major help from Domenic Denicola (Google, d@domenic.me).

Per CC0, to the extent possible under law, the editors have waived all copyright and related or neighboring rights to this work.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[ECMASCRIPT]
ECMAScript Language Specification. URL: https://tc39.github.io/ecma262/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://heycam.github.io/webidl/

IDL Index

[Exposed=(Window,Worker,Worklet)]
namespace console { // but see namespace object requirements below
  // Logging
  void assert(optional boolean condition = false, any... data);
  void clear();
  void count(optional DOMString label = "default");
  void debug(any... data);
  void error(any... data);
  void info(any... data);
  void log(any... data);
  void table(any tabularData, optional sequence<DOMString> properties);
  void trace(any... data);
  void warn(any... data);
  void dir(any item, optional object? options);
  void dirxml(any... data);

  // Grouping
  void group(any... data);
  void groupCollapsed(any... data);
  void groupEnd();

  // Timing
  void time(optional DOMString label = "default");
  void timeEnd(optional DOMString label = "default");
};