console

The console object provides access to the browser's debugging console. The specifics of how it works vary from browser to browser, but there is a de facto set of features that are typically provided.

For details on how this object interacts with the Web Console in Firefox, see Web Console.

Methods

console.debug() Deprecated since Gecko 5.0

An alias for log(); this was added to improve compatibility with existing sites already using debug(). However, you should use console.log() instead.

console.dir()

Displays an interactive listing of the properties of a specified JavaScript object. This listing lets you use disclosure triangles to examine the contents of child objects.

console.error()

Outputs an error message. You may use string substitution and additional arguments with this method; see Using string substitutions.

console.group()

Creates a new inline group, indenting all following output by another level. To move back out a level, call groupEnd(). See Using groups in the console.

console.groupCollapsed()

Creates a new inline group, indenting all following output by another level; unlike group(), this starts with the inline group collapsed, requiring the use of a disclosure button to expand it. To move back out a level, call groupEnd(). See Using groups in the console.

console.groupEnd()

Exits the current inline group. See Using groups in the console.

console.info()

Informative logging information. You may use string substitution and additional arguments with this method; see Using string substitutions.

console.log()

For general output of logging information. You may use string substitution and additional arguments with this method; see Using string substitutions.

console.time()

Starts a timer with a name specified as an input parameter. Up to 10,000 simultaneous timers can run on a given page.

console.timeEnd()

Stops the specified timer and logs the elapsed time in seconds since its start. See Timers.

console.trace()

Outputs a stack trace. See Stack traces.

console.warn()

Outputs a warning message. You may use string substitution and additional arguments with this method; see Using string substitutions.

Outputting text to the console

The most frequently-used feature of the console is logging of text and other data. There are four categories of output you can generate, using the console.log(), console.info(), console.warn(), and console.error() methods. Each of these results in output that's styled differently in the log, and you can use the filtering controls provided by your browser to only view the kinds of output that interest you.

There are two ways to use each of the output methods; you can simply pass in a list of objects whose string representations get concatenated into one string then output to the console, or you can pass in a string containing zero or more substitution strings followed by a list of the objects with which to replace them.

Outputting a single object

The simplest way to use the logging methods is to output a single object:

var someObject = { str: "Some text", id: 5 };
console.log(someObject);

The output looks something like this:

[09:27:13.475] ({str:"Some text", id:5})

Outputting multiple objects

You can also output multiple objects by simply listing them when calling the logging method, like this:

var car = "Dodge Charger";
var someObject = {str:"Some text", id:5}; 
console.info("My first car was a", car, ". The object is: ", someObject);

This output will look like this:

[09:28:22.711] My first car was a Dodge Charger . The object is:  ({str:"Some text", id:5})

Gecko 9.0 (Firefox 9.0 / Thunderbird 9.0 / SeaMonkey 2.6) introduced support for string substitutions. When passing a string to one of the console object's methods that accepts a string, you may use these substitution strings:

Each of these pulls the next argument after the format string off the parameter list. For example:

for (var i=0; i<5; i++) {
  console.log("Hello, %s. You've called me %d times.", "Bob", i+1);
}

The output looks like this:

[13:14:13.481] Hello, Bob. You've called me 1 times.
[13:14:13.483] Hello, Bob. You've called me 2 times.
[13:14:13.485] Hello, Bob. You've called me 3 times.
[13:14:13.487] Hello, Bob. You've called me 4 times.
[13:14:13.488] Hello, Bob. You've called me 5 times.

You can use nested groups to help organize your output by visually combining related material. To create a new nested block, call console.group(). The console.groupCollapsed() method is similar, but creates the new block collapsed, requiring the use of a disclosure button to open it for reading.

To exit the current group, simply call console.groupEnd().

For example, given this code:

console.log("This is the outer level");
console.group();
console.log("Level 2");
console.group();
console.log("Level 3");
console.warn("More of level 3");
console.groupEnd();
console.log("Back to level 2");
console.groupEnd();
console.debug("Back to the outer level");

The output looks like this:

In order to calculate the duration of a specific operation, Gecko 10 introduced the support of timers in the console object. To start a timer, call the console.time() method, giving it a name as only parameter. To stop the timer, and to get the elapsed time in miliseconds, just call the console.timeEnd() method, again passing the timer's name as the parameter. Up to 10,000 timers can run simultaneously on a given page.

For example, given this code:

console.time("answer time");
alert("Click to continue");
console.timeEnd("answer time");

will log the time needed by the user to discard the alert box:

Notice that the timer's name is displayed both when the timer is started and when it's stopped.

Stack traces

The console object also supports outputting a stack trace; this will show you the call path taken to reach the point at which you call console.trace(). Consider this code:

function foo() {
  function bar() {
    console.trace();
  }
}

When you call foo(), the output to the console looks like this:

You'll note that the "Stack trace from Web Console, function bar, line 1" entry in the log is a hyperlink. Clicking on it gives you an object inspector you can use to get more details:

This output provides a lot of handy information you can use to look through your code and figure out how your code got where it is (which is especially nice when it's not supposed to have gotten there in the first place!).

See also

• Tools
• Web Console
• On-device console logging (how to do logging on Firefox OS devices)
• Using the remote web console with Firefox OS