{ "source": "doc/api/console.markdown", "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.\n\n
The module exports two specific components:\n\n
\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
:\n\n
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
\nExample using the Console
class:\n\n
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 ${name}! Danger!`);\n // Prints: Danger Will Robinson! Danger!, to err
\nWhile the API for the Console
class is designed fundamentally around the\nWeb browser console
object, the Console
is Node.js is not intended to\nduplicate the browsers functionality exactly.\n\n
The console functions are synchronous when the destination is a terminal or\na file (to avoid lost messages in case of premature exit) and asynchronous\nwhen the destination is a pipe (to avoid blocking for long periods of time).\n\n
\nIn the following example, stdout is non-blocking while stderr is blocking:\n\n
\n$ node script.js 2> error.log | tee info.log
\nTypically, the distinction between blocking/non-blocking is not important\nunless an application is logging significant amounts of data. High volume\nlogging should use a Console
instance that writes to a pipe.\n\n
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
:\n\n
const Console = require('console').Console;\nconst Console = console.Console;
\n",
"methods": [
{
"textRaw": "console.assert(value[, message][, ...])",
"type": "method",
"name": "assert",
"desc": "A simple assertion test that verifies whether value
is truthy. If it is not,\nan AssertionError
is throw. If provided, the error message
is formatted\nusing [util.format()
][] and used as the error message.\n\n
console.assert(true, 'does nothing');\n // OK\nconsole.assert(false, 'Whoops %s', 'didn\\'t work');\n // AssertionError: Whoops didn't work
\n",
"signatures": [
{
"params": [
{
"name": "value"
},
{
"name": "message",
"optional": true
},
{
"name": "...",
"optional": true
}
]
}
]
},
{
"textRaw": "console.dir(obj[, options])",
"type": "method",
"name": "dir",
"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 that alters certain aspects of the\nformatted string:\n\n
showHidden
- if true
then the object's non-enumerable and symbol\nproperties will be shown too. Defaults to false
.
depth
- tells inspect
how many times to recurse while formatting the\nobject. This is useful for inspecting large complicated objects. Defaults to\n2
. 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\n[customizing util.inspect()
colors][].
Prints to stderr with newline. Multiple arguments can be passed, with the first\nused as the primary message and all additional used as substitution\nvalues similar to printf()
(the arguments are all passed to\n[util.format()
][]).\n\n
const code = 5;\nconsole.error('error #%d', code);\n // Prints: error #5, to stderr\nconsole.error('error', code);\n // Prints: error 5, to stderr
\nIf formatting elements (e.g. %d
) are not found in the first string then\n[util.inspect()
][] is called on each argument and the resulting string\nvalues are concatenated. See [util.format()
][] for more information.\n\n
The console.info()
function is an alias for [console.log()
][].\n\n
Prints to stdout with newline. Multiple arguments can be passed, with the first\nused as the primary message and all additional used as substitution\nvalues similar to printf()
(the arguments are all passed to\n[util.format()
][]).\n\n
var count = 5;\nconsole.log('count: %d', count);\n // Prints: count: 5, to stdout\nconsole.log('count: ', count);\n // Prints: count: 5, to stdout
\nIf formatting elements (e.g. %d
) are not found in the first string then\n[util.inspect()
][] is called on each argument and the resulting string\nvalues are concatenated. See [util.format()
][] for more information.\n\n
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\n[console.timeEnd()
][] method, again passing the\ntimer's unique label
as the parameter.\n\n
Stops a timer that was previously started by calling [console.time()
][] and\nprints the result to stdout:\n\n
console.time('100-elements');\nfor (var i = 0; i < 100; i++) {\n ;\n}\nconsole.timeEnd('100-elements');\n// prints 100-elements: 262ms
\n",
"signatures": [
{
"params": [
{
"name": "label"
}
]
}
]
},
{
"textRaw": "console.trace(message[, ...])",
"type": "method",
"name": "trace",
"desc": "Prints to stderr the string 'Trace :'
, followed by the [util.format()
][]\nformatted message and stack trace to the current position in the code.\n\n
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",
"signatures": [
{
"params": [
{
"name": "message"
},
{
"name": "...",
"optional": true
}
]
}
]
},
{
"textRaw": "console.warn([data][, ...])",
"type": "method",
"name": "warn",
"desc": "The console.warn()
function is an alias for [console.error()
][].\n\n
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, the warning\nand error output will be sent to the stdout
.\n\n
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\nvar count = 5;\nlogger.log('count: %d', count);\n// in stdout.log: count 5
\nThe global console
is a special Console
whose output is sent to\nprocess.stdout
and process.stderr
. It is equivalent to calling:\n\n
new Console(process.stdout, process.stderr);
\n"
}
]
}
],
"type": "module",
"displayName": "Console"
}
]
}