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. Supporting abstract operations
1.1. Logger(logLevel, args)
The logger operation accepts a log level and a
- If args is empty, abort these steps.
- Let first be the first element of args.
- Let rest be all elements following first in args.
- If rest is empty, perform
Printer (logLevel, «first»). Abort these steps. - If first does not contain any format specifiers, perform
Printer (logLevel, args). - Otherwise, perform
Printer (logLevel,Formatter (args)). - Return
undefined .
console.log("hello!")
, this should first print "hello!", then the undefined return value from the console.log call.
1.2. 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
- Let target be the first element of args.
- Let current be the second element of args.
- Find the first possible format specifier specifier, from the left to the right in target.
- If specifier is
%s
, let converted be the result ofToString (current). - If specifier is
%d
or%i
, let converted be the result of%parseInt% (current, 10). - If specifier is
%f
, let converted be the result of%parseFloat% (current, 10). - If specifier is
%o
, optionally let converted be an implementation-specific representation of current that is judged to be maximally useful and informative. - If specifier is
%O
, optionally let converted be be an implementation-specific representation of current as an expanded or expandable object, treating current as a generic JavaScript object. TODO: process %c
- If any of the previous steps set converted, replace specifier in target with converted.
- Let result be a
List containing target together with the elements of args starting from the third onward.
- If specifier is
- If target does not have any format specifiers left, return result.
- If result contains just one element, return result.
- Return
Formatter (result).
1.2.1. 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 | |
%d or %i
| Element which substitutes is converted to an integer | |
%f
| Element which substitutes is converted to a float | |
%o
| Element is displayed in an implementation-specific way judged as most useful; potentially interactive | n/a |
%O
| Element is displayed as an expanded JavaScript object; potentially interactive | n/a |
%c
| Applies provided CSS | n/a |
1.3. Printer(logLevel, args)
The printer operation is implementation-defined. It accepts a log level indicating severity, and a %o
and %O
specifiers). 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
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).
1.3.1. Example printer in Node.js
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
2. 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 = ""); 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); // 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
2.1. Logging methods
2.1.1. assert(condition, ...data)
If condition is false, perform
2.1.2. clear()
If possible for the environment, clear the console. Otherwise, do nothing.
2.1.3. count(label)
2.1.4. debug(...data)
Perform
2.1.5. error(...data)
Perform
2.1.6. info(...data)
Perform
2.1.7. log(...data)
Perform
2.1.8. 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.
2.1.9. trace(...data)
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
2.1.10. warn(...data)
Perform
2.2. Grouping methods
2.2.1. group(...data)
2.2.2. groupCollapsed(...data)
2.2.3. groupEnd()
2.3. Timing methods
2.3.1. time(label)
Start an internal timer stored in the timer table with key label.
2.3.2. timeEnd(label)
Let duration be the current value of the internal timer with key label in the timer table.
Remove the timer from the timer table.
Then, perform
3. JavaScript object inspection
TODO: Define an interface that allows JavaScript objects to format themselves for inspection.
Acknowledgments
The editors would like to thank Boris Zbarsky, Brent S.A. Cowgill, Brian Grinstead, Corey Farwell, Dominic Farolino, 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) and Robert Kowalski (rok@kowalski.gd) 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.