{ "source": "doc/api/console.md", "modules": [ { "textRaw": "Console", "name": "console", "stability": 2, "stabilityText": "Stable", "desc": "
The console
module provides a simple debugging console that is similar to the\nJavaScript console mechanism provided by web browsers.
The module exports two specific components:
\nConsole
class with methods such as console.log()
, console.error()
and\nconsole.warn()
that can be used to write to any Node.js stream.console
instance configured to write to stdout
and stderr
.\nBecause this object is global, it can be used without calling\nrequire('console')
.Example using the global console
:
console.log('hello world');\n // Prints: hello world, to stdout\nconsole.log('hello %s', 'world');\n // Prints: hello world, to stdout\nconsole.error(new Error('Whoops, something bad happened'));\n // Prints: [Error: Whoops, something bad happened], to stderr\n\nconst name = 'Will Robinson';\nconsole.warn(`Danger ${name}! Danger!`);\n // Prints: Danger Will Robinson! Danger!, to stderr\n
\nExample using the Console
class:
const out = getStreamSomehow();\nconst err = getStreamSomehow();\nconst myConsole = new console.Console(out, err);\n\nmyConsole.log('hello world');\n // Prints: hello world, to out\nmyConsole.log('hello %s', 'world');\n // Prints: hello world, to out\nmyConsole.error(new Error('Whoops, something bad happened'));\n // Prints: [Error: Whoops, something bad happened], to err\n\nconst name = 'Will Robinson';\nmyConsole.warn(`Danger ${name}! Danger!`);\n // Prints: Danger Will Robinson! Danger!, to err\n
\nWhile the API for the Console
class is designed fundamentally around the\nbrowser console
object, the Console
in Node.js is not intended to\nduplicate the browser's functionality exactly.
The console functions are usually asynchronous unless the destination is a file.\nDisks are fast and operating systems normally employ write-back caching;\nit should be a very rare occurrence indeed that a write blocks, but it\nis possible.
\nAdditionally, console functions are blocking when outputting to TTYs\n(terminals) on OS X as a workaround for the OS's very small, 1kb buffer size.\nThis is to prevent interleaving between stdout
and stderr
.
The Console
class can be used to create a simple logger with configurable\noutput streams and can be accessed using either require('console').Console
\nor console.Console
:
const Console = require('console').Console;\nconst Console = console.Console;\n
\n",
"methods": [
{
"textRaw": "console.assert(value[, message][, ...])",
"type": "method",
"name": "assert",
"meta": {
"added": [
"v0.1.101"
]
},
"desc": "A simple assertion test that verifies whether value
is truthy. If it is not,\nan AssertionError
is thrown. If provided, the error message
is formatted\nusing util.format()
and used as the error message.
console.assert(true, 'does nothing');\n // OK\nconsole.assert(false, 'Whoops %s', 'didn\\'t work');\n // AssertionError: Whoops didn't work\n
\nNote: the console.assert()
method is implemented differently in Node.js\nthan the console.assert()
method available in browsers.
Specifically, in browsers, calling console.assert()
with a falsy\nassertion will cause the message
to be printed to the console without\ninterrupting execution of subsequent code. In Node.js, however, a falsy\nassertion will cause an AssertionError
to be thrown.
Functionality approximating that implemented by browsers can be implemented\nby extending Node.js' console
and overriding the console.assert()
method.
In the following example, a simple module is created that extends and overrides\nthe default behavior of console
in Node.js.
'use strict';\n\n// Creates a simple extension of console with a\n// new impl for assert without monkey-patching.\nconst myConsole = Object.create(console, {\n assert: {\n value: function assert(assertion, message, ...args) {\n try {\n console.assert(assertion, message, ...args);\n } catch (err) {\n console.error(err.stack);\n }\n },\n configurable: true,\n enumerable: true,\n writable: true,\n },\n});\n\nmodule.exports = myConsole;\n
\nThis can then be used as a direct replacement for the built in console:
\nconst console = require('./myConsole');\nconsole.assert(false, 'this message will print, but no error thrown');\nconsole.log('this will also print');\n
\n",
"signatures": [
{
"params": [
{
"name": "value"
},
{
"name": "message",
"optional": true
},
{
"name": "...",
"optional": true
}
]
}
]
},
{
"textRaw": "console.dir(obj[, options])",
"type": "method",
"name": "dir",
"meta": {
"added": [
"v0.1.101"
]
},
"desc": "Uses util.inspect()
on obj
and prints the resulting string to stdout
.\nThis function bypasses any custom inspect()
function defined on obj
. An\noptional options
object may be passed to alter certain aspects of the\nformatted string:
showHidden
- if true
then the object's non-enumerable and symbol\nproperties will be shown too. Defaults to false
.
depth
- tells util.inspect()
how many times to recurse while\nformatting the object. This is useful for inspecting large complicated objects.\nDefaults to 2
. To make it recurse indefinitely, pass null
.
colors
- if true
, then the output will be styled with ANSI color codes.\nDefaults to false
. Colors are customizable; see\ncustomizing util.inspect()
colors.
Prints to stderr
with newline. Multiple arguments can be passed, with the\nfirst used as the primary message and all additional used as substitution\nvalues similar to printf(3) (the arguments are all passed to\nutil.format()
).
const code = 5;\nconsole.error('error #%d', code);\n // Prints: error #5, to stderr\nconsole.error('error', code);\n // Prints: error 5, to stderr\n
\nIf formatting elements (e.g. %d
) are not found in the first string then\nutil.inspect()
is called on each argument and the resulting string\nvalues are concatenated. See util.format()
for more information.
The console.info()
function is an alias for console.log()
.
Prints to stdout
with newline. Multiple arguments can be passed, with the\nfirst used as the primary message and all additional used as substitution\nvalues similar to printf(3) (the arguments are all passed to\nutil.format()
).
const count = 5;\nconsole.log('count: %d', count);\n // Prints: count: 5, to stdout\nconsole.log('count:', count);\n // Prints: count: 5, to stdout\n
\nIf formatting elements (e.g. %d
) are not found in the first string then\nutil.inspect()
is called on each argument and the resulting string\nvalues are concatenated. See util.format()
for more information.
Used to calculate the duration of a specific operation. To start a timer, call\nthe console.time()
method, giving it a unique label
as the only parameter. To stop the\ntimer, and to get the elapsed time in milliseconds, just call the\nconsole.timeEnd()
method, again passing the\ntimer's unique label
as the parameter.
Stops a timer that was previously started by calling console.time()
and\nprints the result to stdout:
console.time('100-elements');\nfor (let i = 0; i < 100; i++) {\n ;\n}\nconsole.timeEnd('100-elements');\n// prints 100-elements: 262ms\n
\n",
"signatures": [
{
"params": [
{
"name": "label"
}
]
}
]
},
{
"textRaw": "console.trace(message[, ...])",
"type": "method",
"name": "trace",
"meta": {
"added": [
"v0.1.104"
]
},
"desc": "Prints to stderr
the string 'Trace :'
, followed by the util.format()
\nformatted message and stack trace to the current position in the code.
console.trace('Show me');\n // Prints: (stack trace will vary based on where trace is called)\n // Trace: Show me\n // at repl:2:9\n // at REPLServer.defaultEval (repl.js:248:27)\n // at bound (domain.js:287:14)\n // at REPLServer.runBound [as eval] (domain.js:300:12)\n // at REPLServer.<anonymous> (repl.js:412:12)\n // at emitOne (events.js:82:20)\n // at REPLServer.emit (events.js:169:7)\n // at REPLServer.Interface._onLine (readline.js:210:10)\n // at REPLServer.Interface._line (readline.js:549:8)\n // at REPLServer.Interface._ttyWrite (readline.js:826:14)\n
\n",
"signatures": [
{
"params": [
{
"name": "message"
},
{
"name": "...",
"optional": true
}
]
}
]
},
{
"textRaw": "console.warn([data][, ...])",
"type": "method",
"name": "warn",
"meta": {
"added": [
"v0.1.100"
]
},
"desc": "The console.warn()
function is an alias for console.error()
.
Creates a new Console
by passing one or two writable stream instances.\nstdout
is a writable stream to print log or info output. stderr
\nis used for warning or error output. If stderr
isn't passed, warning and error\noutput will be sent to stdout
.
const output = fs.createWriteStream('./stdout.log');\nconst errorOutput = fs.createWriteStream('./stderr.log');\n// custom simple logger\nconst logger = new Console(output, errorOutput);\n// use it like console\nconst count = 5;\nlogger.log('count: %d', count);\n// in stdout.log: count 5\n
\nThe global console
is a special Console
whose output is sent to\nprocess.stdout
and process.stderr
. It is equivalent to calling:
new Console(process.stdout, process.stderr);\n
\n"
}
]
}
],
"type": "module",
"displayName": "Console"
}
]
}