Stability: 2 - Stable
-The module defines a `Console` class and exports a `console` object.
+The `console` module provides a simple debugging console that is similar to the
+JavaScript console mechanism provided by web browsers.
-The `console` object is a special instance of `Console` whose output is
-sent to stdout or stderr.
+The module exports two specific components:
-For ease of use, `console` is defined as a global object and can be used
-directly without `require`.
+* A `Console` class with methods such as `console.log()`, `console.error()` and
+ `console.warn()` that can be used to write to any Node.js stream.
+* A global `console` instance configured to write to `stdout` and `stderr`.
+ Because this object is global, it can be used without calling
+ `require('console')`.
+
+Example using the global `console`:
+
+ console.log('hello world');
+ // Prints: hello world, to stdout
+ console.log('hello %s', 'world');
+ // Prints: hello world, to stdout
+ console.error(new Error('Whoops, something bad happened'));
+ // Prints: [Error: Whoops, something bad happened], to stderr
+
+ const name = 'Will Robinson';
+ console.warn(`Danger ${name}! Danger!`);
+ // Prints: Danger Will Robinson! Danger!, to stderr
+
+Example using the `Console` class:
+
+ const out = getStreamSomehow();
+ const err = getStreamSomehow();
+ const myConsole = new console.Console(out, err);
+
+ myConsole.log('hello world');
+ // Prints: hello world, to out
+ myConsole.log('hello %s', 'world');
+ // Prints: hello world, to out
+ myConsole.error(new Error('Whoops, something bad happened'));
+ // Prints: [Error: Whoops, something bad happened], to err
+
+ const name = 'Will Robinson';
+ myConsole.warn(`Danger ${name}! Danger!`);
+ // Prints: Danger Will Robinson! Danger!, to err
+
+While the API for the `Console` class is designed fundamentally around the
+Web browser `console` object, the `Console` is Node.js is *not* intended to
+duplicate the browsers functionality exactly.
+
+## Asynchronous vs Synchronous Consoles
+
+The console functions are synchronous when the destination is a terminal or
+a file (to avoid lost messages in case of premature exit) and asynchronous
+when the destination is a pipe (to avoid blocking for long periods of time).
+
+In the following example, stdout is non-blocking while stderr is blocking:
+
+ $ node script.js 2> error.log | tee info.log
+
+Typically, the distinction between blocking/non-blocking is not important
+unless an application is logging significant amounts of data. High volume
+logging *should* use a `Console` instance that writes to a pipe.
## Class: Console
<!--type=class-->
-Use `require('console').Console` or `console.Console` to access this class.
+The `Console` class can be used to create a simple logger with configurable
+output streams and can be accessed using either `require('console').Console`
+or `console.Console`:
const Console = require('console').Console;
const Console = console.Console;
-You can use the `Console` class to create a simple logger like `console` but
-with different output streams.
-
### new Console(stdout[, stderr])
-Create a new `Console` by passing one or two writable stream instances.
+Creates a new `Console` by passing one or two writable stream instances.
`stdout` is a writable stream to print log or info output. `stderr`
is used for warning or error output. If `stderr` isn't passed, the warning
and error output will be sent to the `stdout`.
// in stdout.log: count 5
The global `console` is a special `Console` whose output is sent to
-`process.stdout` and `process.stderr`:
+`process.stdout` and `process.stderr`. It is equivalent to calling:
new Console(process.stdout, process.stderr);
-## console
-
-* {Object}
-
-<!--type=global-->
-
-For printing to stdout and stderr. Similar to the console object functions
-provided by most web browsers, here the output is sent to stdout or stderr.
-
-The console functions are synchronous when the destination is a terminal or
-a file (to avoid lost messages in case of premature exit) and asynchronous
-when it's a pipe (to avoid blocking for long periods of time).
-
-That is, in the following example, stdout is non-blocking while stderr
-is blocking:
-
- $ node script.js 2> error.log | tee info.log
-
-Typically, the blocking/non-blocking dichotomy is not something you should
-worry about unless you log huge amounts of data.
-
### console.assert(value[, message][, ...])
-Similar to [`assert.ok()`][], but the error message is formatted as
-`util.format(message...)`.
+A simple assertion test that verifies whether `value` is truthy. If it is not,
+an `AssertionError` is throw. If provided, the error `message` is formatted
+using [`util.format()`][] and used as the error message.
+
+ console.assert(true, 'does nothing');
+ // OK
+ console.assert(false, 'Whoops %s', 'didn\'t work');
+ // AssertionError: Whoops didn't work
### console.dir(obj[, options])
Uses [`util.inspect()`][] on `obj` and prints the resulting string to stdout.
-This function bypasses any custom `inspect()` function on `obj`. An optional
-`options` object may be passed that alters certain aspects of the formatted
-string:
+This function bypasses any custom `inspect()` function defined on `obj`. An
+optional `options` object may be passed that alters certain aspects of the
+formatted string:
- `showHidden` - if `true` then the object's non-enumerable and symbol
properties will be shown too. Defaults to `false`.
### console.error([data][, ...])
-Same as [`console.log()`][] but prints to stderr.
+Prints to stderr with newline. Multiple arguments can be passed, with the first
+used as the primary message and all additional used as substitution
+values similar to `printf()` (the arguments are all passed to
+[`util.format()`][]).
+
+ const code = 5;
+ console.error('error #%d', code);
+ // Prints: error #5, to stderr
+ console.error('error', code);
+ // Prints: error 5, to stderr
+
+If formatting elements (e.g. `%d`) are not found in the first string then
+[`util.inspect()`][] is called on each argument and the resulting string
+values are concatenated. See [`util.format()`][] for more information.
### console.info([data][, ...])
-Same as [`console.log()`][].
+The `console.info()` function is an alias for [`console.log()`][].
### console.log([data][, ...])
-Prints to stdout with newline. This function can take multiple arguments in a
-`printf()`-like way:
+Prints to stdout with newline. Multiple arguments can be passed, with the first
+used as the primary message and all additional used as substitution
+values similar to `printf()` (the arguments are all passed to
+[`util.format()`][]).
var count = 5;
console.log('count: %d', count);
- // prints 'count: 5'
+ // Prints: count: 5, to stdout
+ console.log('count: ', count);
+ // Prints: count: 5, to stdout
-If formatting elements are not found in the first string then
-[`util.inspect()`][] is used on each argument. See [`util.format()`][] for more
-information.
+If formatting elements (e.g. `%d`) are not found in the first string then
+[`util.inspect()`][] is called on each argument and the resulting string
+values are concatenated. See [`util.format()`][] for more information.
### console.time(label)
Used to calculate the duration of a specific operation. To start a timer, call
-the `console.time()` method, giving it a name as only parameter. To stop the
+the `console.time()` method, giving it a unique `label` as the only parameter. To stop the
timer, and to get the elapsed time in milliseconds, just call the
[`console.timeEnd()`][] method, again passing the
-timer's name as the parameter.
+timer's unique `label` as the parameter.
### console.timeEnd(label)
Stops a timer that was previously started by calling [`console.time()`][] and
-prints the result to the console:
+prints the result to stdout:
console.time('100-elements');
for (var i = 0; i < 100; i++) {
### console.trace(message[, ...])
-Print to stderr `'Trace :'`, followed by the formatted message and stack trace
-to the current position.
+Prints to stderr the string `'Trace :'`, followed by the [`util.format()`][]
+formatted message and stack trace to the current position in the code.
+
+ console.trace('Show me');
+ // Prints: (stack trace will vary based on where trace is called)
+ // Trace: Show me
+ // at repl:2:9
+ // at REPLServer.defaultEval (repl.js:248:27)
+ // at bound (domain.js:287:14)
+ // at REPLServer.runBound [as eval] (domain.js:300:12)
+ // at REPLServer.<anonymous> (repl.js:412:12)
+ // at emitOne (events.js:82:20)
+ // at REPLServer.emit (events.js:169:7)
+ // at REPLServer.Interface._onLine (readline.js:210:10)
+ // at REPLServer.Interface._line (readline.js:549:8)
+ // at REPLServer.Interface._ttyWrite (readline.js:826:14)
### console.warn([data][, ...])
-Same as [`console.error()`][].
+The `console.warn()` function is an alias for [`console.error()`][].
-[`assert.ok()`]: assert.html#assert_assert_value_message_assert_ok_value_message
[`console.error()`]: #console_console_error_data
[`console.log()`]: #console_console_log_data
[`console.time()`]: #console_console_time_label
[`console.timeEnd()`]: #console_console_timeend_label
[`util.format()`]: util.html#util_util_format_format
[`util.inspect()`]: util.html#util_util_inspect_object_options
-[customizing `util.inspect()` colors]: util.html#util_customizing_util_inspect_colors
projects / platform / upstream / nodejs.git / commitdiff