ওয়েব কনসোল

The Web Console:

1. logs information associated with a web page: any network requests, JavaScript, CSS, and security errors and warnings, as well as error, warning, and informational messages explicitly logged by JavaScript code running in the page context
2. enables you to interact with a web page by executing JavaScript expressions in the context of the page

It's part of the replacement for the old Error Console built into Firefox: but the Error Console showed errors, warnings, and messages from all web pages, from the browser's own code, and from add-ons. This makes it much more difficult to see which messages are relevant to a specific page. The Web Console is always associated with a specific web page and only shows information associated with that page.

The other half of the replacement for the Error console is the Browser Console, which shows errors, warnings, and messages from the browser's code and from add-ons.

ওয়েব কনসোল চালু করা

To open the Web Console select "Web Console" from the Web Developer submenu in the Firefox Menu (or Tools menu if you display the menu bar or are on Mac OS X), or by pressing its Control-Shift-K (Command-Option-K on the Mac) keyboard shortcut.

The Toolbox will appear at the bottom of the browser window, with the Web Console activated (it's just called "Console" in the DevTools toolbar):

Underneath the DevTools Window's own toolbar, the Web Console's interface is split into three sections:

• Toolbar: along the top is a toolbar containing buttons with labels like "Net", "CSS", and "JS". This toolbar is used to filter which messages are displayed
• Command Line: along the bottom is a command line that you can use to enter JavaScript expressions
• Message Display Pane: in between the toolbar and the command line, and occupying most of the window, is the space in which the Web Console displays messages

মেসেজ ডিসপ্লে প্যানেল

Most of the Web Console is occupied by the message display pane:

The message display pane displays the following sorts of messages:

• HTTP requests
• JavaScript warnings and errors
• CSS warnings, errors, and reflow events
• Security warnings and errors
• console API calls
• Input/output messages: commands send to the browser via the Web Console's command line, and the result of executing them.

Each message is displayed as a separate row:

HTTP requests

HTTP requests are logged with a line that looks like this:

If you click on the message, you'll see a window like this, containing more details about the request and response:

Scrolling down reveals the response headers. By default, the Web Console does not log request and response bodies: to do this, activate the context menu in the Web Console and select "Log Request and Response Bodies", reload the page, and you'll then see them in the "Inspect Network Request" window.

Only the first megabyte of data is logged for each request or response body, so very large requests and responses will be truncated.

জাভাস্ক্রিপ্টের ভুল ও ওয়ার্নিং

JavaScript messages look like this:

সিএসএস ভুল, ওয়ার্নিং এবং রিফ্লো মেসেজ সমূহ

CSS messages look like this:

By default, CSS warnings and log messages are not shown.

রিফ্লো ইভেন্ট সমূহ

The Web Console also logs reflow events under the CSS category. A reflow is the name given to the operation in which the browser calculates the layout of all or part of the page. Reflows occur when a change has happened to a page that the browser thinks affects the layout. Many events can trigger reflows, including: resizing the browser window, activating pseudoclasses like :hover, or manipulating the DOM in JavaScript.

Because reflows can be computationally expensive and directly affect the user interface, they can have a big impact on the responsiveness of a website or web app. By logging reflow events the Web Console can give you insight into when reflow events are being triggered, how long they take to execute and, if the reflows are synchronous reflows triggered from JavaScript, which code triggered them.

Reflow events are logged as "Log" messages, as distinct from CSS errors or warnings. By default, they are disabled. You can enable them by clicking the "CSS" button in the toolbar and selecting "Log".

Each message is labeled "reflow" and shows the time taken to execute the reflow:

If the reflow is a synchronous reflow triggered from JavaScript, it also shows a link to the line of code that triggered the reflow:

Click the link to open the file in the Debugger.

সিনক্রোনাস ও এসিনক্রোনাস রিফ্লো সমূহ

If a change is made that invalidates the current layout - for example, the browser window is resized or some JavaScript modifies an element's CSS - the layout is not recalculated immediately. Instead, the reflow happens asynchronously, the next time the browser decides it needs to be done (generally, the next time the browser repaints). In this way the browser can save up a collection of invalidating changes and recalculate their effect at once.

However, if some JavaScript code reads a style that has been modified, then the browser must perform a synchronous reflow in order to calculate the computed style to return. For example, code like this will cause an immediate, synchronous, reflow, when it calls window.getComputedStyle(thing).height:

var thing = document.getElementById("the-thing");
thing.style.display = "inline-block";
var thingHeight = window.getComputedStyle(thing).height;

Because of this, it's a good idea to avoid interleaving write and read calls to an element's styles when manipulating the DOM, because every time you read back a style that has been invalidated by a previous write call, you force a synchronous reflow.

সিকিউরিটি ওয়ার্নিং ও ভুল সমূহ

Security warnings and errors look like this:

The security messages shown in the Web Console help developers find potential or actual vulnerabilities in their sites. Additionally, many of these messages help educate developers because they end with a “Learn More” link that takes you to a page with background information and advice for mitigating the issue.

The complete list of security messages is as follows:

Bug 863874 is the meta-bug for logging relevant security messages to the Web Console. If you have more ideas for useful features like the ones discussed here, or are interested in contributing, check out the metabug and its dependencies.

কনসোল এপিআই মেসেজ সমূহ

This section describes the Web Console output for those console API calls that actually result in output. For more general documentation on the console API, please refer to its documentation page.

ইরোর মেসেজ সমূহ

API	Message content
error()	The argument to error(). console.error("an error");
exception()	An alias for error().
assert()	New in Firefox 29. If the assertion succeeds, nothing. If the assertion fails, the argument: console.assert(false, "My assertion always fails");

Warning messages

API	Message content
warn()	The argument to warn(). console.warn("a warning");

Info messages

API	Message content
info()	The argument to info(). console.info("some info");

Log messages

API	Message content
count()	This message is new in Firefox 30. The label supplied, if any, and the number of times this occurrence of count() has been called with the given label: console.count(user.value);
log()	The argument to log(). console.log("logged");
trace()	Stack trace: console.trace();
dir()	Listing of the object's properties: var user = document.getElementById('user'); console.dir(user);
time()	Notification that the specified timer started. console.time("t");
timeEnd()	Duration for the specified timer. console.timeEnd("t");

Grouping messages

You can use console.group() to create indented groups in the console output. See Using groups in the console for more information on this.

Input/output messages

Commands sent to the browser using the Web Console's command line, and the corresponding responses, are logged using lines like this:

The dark gray bar indicates that these are input/output messages, while the direction of the arrow discriminates between input and output.

Filtering and searching

You can use the toolbar along the top to constrain the results displayed.

You can display only specific types of messages or only message containing specific strings.

Finally, you can use this toolbar to clear the log.

The command line interpreter

You can interpret JavaScript expressions in real time using the command line provided by the Web Console.

Entering expressions

To enter expressions just type into the command line and press "Enter". To enter multiline expressions, use "Shift+Enter" instead of "Enter".

The expression you type is echoed in the message display window, followed by the result:

Accessing variables

You can access variables defined in the page, both built-in variables like window and variables added by JavaScript like jQuery:

Autocomplete

The command line has autocomplete: enter the first few letters and a popup appears with possible completions:

Type "Enter" or "Tab" to accept the suggestion, use the up/down arrows to move to a different suggestion, or just keep typing if you don't like any of the suggestions.

The console suggests completions from the scope of the currently executing stack frame. This means that if you've hit a breakpoint in a function you get autocomplete for objects local to the function.

From Firefox 30, you get autocomplete suggestions for array elements, as well:

Inspecting objects

If the result object is an object it appears in italics. Click on it, and you'll see a new panel appear containing details of the object:

To dismiss this panel press "Escape".

Defining variables

You can define your own variables, and then access them:

Highlighting and inspecting nodes

If you hover the mouse over any DOM element in the console output, it's highlighted in the page:

In the screenshot above you'll also see a blue "target" icon next to the node in the console output: click it to switch to the Inspector with that node selected.

Command history

The command line remembers commands you've typed: to move back and forward through your history, use the up and down arrows.

Working with iframes

If a page contains embedded iframes, you can use the cd() command to change the console's scope to a specific iframe, and then you can execute functions defined in the document hosted by that iframe. There are three ways to select an iframe using cd():

You can pass the iframe DOM element:

var frame = document.getElementById("frame1");
cd(frame);

You can pass a CSS selector that matches the iframe:

cd("#frame1");

You can pass the iframe's global window object:

var frame = document.getElementById("frame1");
cd(frame.contentWindow);

To switch the context back to the top-level window, call cd() with no arguments:

cd();

For example, suppose we have a document that embeds an iframe:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
  </head>
  <body>
    <iframe id="frame1" src="static/frame/my-frame1.html"></iframe>
  </body>
</html>

The iframe defines a new function:

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <script>
      function whoAreYou() {
        return "I'm frame1";
      }
   </script>
  </head>
  <body>
  </body>
</html>

You can switch context to the iframe like this:

cd("#frame1");

Now you'll see that the global window's document is the iframe:

And you can call the function defined in the iframe:

Helper commands

The JavaScript command line provided by the Web Console offers a few built-in helper functions that make certain tasks easier.

$()

Looks up a CSS selector string, returning the first element that matches. Equivalent to document.querySelector() or calls the $ function in the page, if it exists.

$$()

Looks up a CSS selector string, returning a list of DOM nodes that match. This is a shortcut for document.querySelectorAll().

$0

The currently-inspected element in the page.

$_

New in Firefox 39. Stores the result of the last expression executed in the console's command line. For example, if you type "2+2 <enter>", then "$_ <enter>", the console will print 4.

$x()

Evaluates an XPath expression and returns an array of matching nodes.

keys()

Given an object, returns a list of the keys (or property names) on that object. This is a shortcut for Object.keys.

values()

Given an object, returns a list of the values on that object; serves as a companion to keys().

clear()

Clears the console output area.

inspect()

Given an object, opens the object inspector for that object.

pprint()

Formats the specified value in a readable way; this is useful for dumping the contents of objects and arrays.

help()

Displays help text. Actually, in a delightful example of recursion, it will bring you to this page.

cd()

Switch JavaScript evaluation context to a different iframe in the page. See working with iframes.

copy()

New in Firefox 38. Copy the argument to the clipboard. If the argument is a string, it's copied as-is. If the argument is a DOM node, its outerHTML is copied. Otherwise, JSON.stringify will be called on the argument, and the result will be copied to the clipboard.

clearHistory()

New in Firefox 39. Just like a normal command line, the console command line remembers the commands you've typed. Use this function to clear the console's command history.

Please refer to the Console API for more information about logging from content.

The split console

Starting in Firefox 28, you can use the console alongside other tools. While you're in another tool in the Toolbox, just press "Escape" or press the "Toggle split console" button in the Toolbar. The toolbox will now appear split, with the original tool above and the web console underneath.

As usual, $0 works as a shorthand for the element currently selected in the Inspector:

When you use the split console with the debugger, the console's scope is the currently executing stack frame. So if you hit a breakpoint in a function, the scope will be the function's scope. You'll get autocomplete for objects defined in the function, and can easily modify them on the fly:

Keyboard shortcuts

Command line interpreter

These shortcuts apply when you're in the command line interpreter.

Autocomplete popup

These shortcuts apply while the autocomplete popup is open:

Global shortcuts

These shortcuts work in all tools that are hosted in the toolbox.