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Learn important console object in javascript


Console Object logging

During the development environment, the Developer needs to log the useful debug messages and print flow variable values. So the application has to access the browser window console functionality.

For that, javascript provided Console object to log the messages to the console window.

Console object behavior can be varied from browser to browser.

All popular browsers support the major functionality of the console object.

This object is a global object and can be accessed freely anywhere.

all the information from the console object is output to the console window in a browser.

How to open the Browser console window?

the console window can be enabled in all popular browsers.

A chrome console window will be opened by right click ( more tools + developer tools) on the browser and selecting the Inspect option. the window opened and select the console tab as shown below
chrome console window inspect

Firefox browser console window_will be opened using Firebug extension or inbuilt in_developer tools_ console.

For Safari browser, Please see Safari Developer tools article.

Console Object log syntax

console.log(  object1,{optional objects});

console.log(  message1,{optional objects});

console.log is in two variants. one is having parameters that contain a list of objects and the first object is mandatory. other is having messages followed by zero or more objects.

Console Object methods

console object has a lot of methods for debugging logs/different logger formats/ display timer information.

The assert() method

Print log messages to console if the assert is false, if true, returns nothing. It is useful in unit testing javascript objects.

Syntax

console.assert(assertion, {List of objects, Atleast One is required); console.assert(assertion, messages {list of options/objects});

Example

Here is an example of the console assert method

console.assert(false, "test passed");  
// outputs "VM339:1 Assertion failed: test passed" message colored in red to console
console.assert(true, "test failed"); // outputs nothing to console

The clear() method

this method clears the console messages.

if the preserve option is set, outputs ‘console.clear() was prevented due to ‘Preserve log’’ to console window if the preserve option is not set and the output message “console was cleared”

Syntax and Example:-

console clear syntax and example is below

console.clear()

The count() method usage

this logs the number of times the count is being called.

Syntax :-

console.count(\[label\]); // syntax has label which is optional

Example:-

console.count('calledcount'); // outputs  calledcount:1 

console.count('calledcount'); // outputs  calledcount:2


console.count('calledcount'); // outputs  calledcount:3

The dir() method usage

Output javascript object and their properties information in hierarchical tree(childs) form

Syntax and Example

console.dir(object);  // Syntax
console.dir(document.body); // outputs body object data in tree form as shown in below diagram

Outputs:

console dir method example

The dirxml() method usage

prints javascript object data in the form of XML/HTML to console window.

Syntax and Example:-

console.dirxml(object);
console.dirxml(document.body); // outputs body object data in html form as shown in below diagram.
console object dirxml method

Group(), groupEnd() and GroupCollapsed() method usage

these methods are used to group the console log messages and show data in hierarchical format as shown below.

You can create a nested group inside the group using the group()method. groupCollapse creates a new group + opens the group by default

Syntax and Example:-

console.log("My Computer");

console.group();

console.log("C-Drive");

console.log("c-drive-file");

console.groupEnd();

console.group();

console.log("D-drive");

console.warn("D-drive-File");

console.groupEnd();

console.log("File-2");

console.log("File-3");
console object group methods

The table() method usage

Displays the data in table format. This will be useful for printing array of objects to console.

Syntax

console.table(data \[, columns\]);

Parameters:

  • data:- data array or object data to print
  • columns: column array to include in the output

Example

  
function Employee(name, salary) {  
  this.name = name;  
  this.salary= salary;  
}  
var emp1 = new Employee("Sai", "9000");  
var emp2 = new Employee("Ganesh", "8000");  
var emp3 = new Employee("Venkat", "5000");  
  
console.table([emp1,emp2,emp3]);  
  

please see the screenshot form output how it looks

console Object table method example

time(), TimeEnd() methods

This method is used to log the time taken for operations/calculation processing.
time() method initializes and starts the timer when timeEnd encountered, the timer stops and logs the time taken.

console info() method usage

log information message to the Web console

The trace() method

trace method logs a stack trace of the current request thread.

syntax

console.trace()

The warn() method

prints warning messages to the web console

Syntax

console.warn(Obj1,\[list of objects\])

console.warn(msg,\[list of objects\])

The Error() method usage

Prints Error messages to the web console. The syntax is similar to the log method

Following 5 log messages, types are in order from left to right where Left is termed as a critical message and the Right one is termed as Low.

FATAL –> ERROR –> WARN –>INFO –>DEBUG

Conclusion

Console object is an important native object in javascript. It is helpful to print the debug logs to the browser console.

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