JavaScript Console

Console

JavaScript Console or Browser console is build-in object in JavaScript which gives access to browser debugging console. console can be used to print output, check errors and warnings in console.

For frontend and web developer, its is recommended to open console window always. We can call global functions, check variables and syntax errors in console.

Open Browser Console

console.log()

console.log is used to print output in browser console. We can print string or any others datatype.

console is build-in object and log is method of console.

print in console

    console.log("Hello JS");           
        

Print variable in console

    var fname="Tech";         
    var lname="Altum";
    console.log(fname,lname);         
        

Placeholder in console

We can also use Placeholder like %s, %d to insert string or numbers.

String Placeholder

    var name="Tech Altum";         
    console.log("Name is %s",name);         
        

Number Placeholder

    var id=12;         
    console.log("Id is %d",id);         
        

Style console output

To add style in console, use %c and add css property:value in second argument.

    console.log("This color is %cred","color:red");    
    

    console.log("This text is %chighlighted text","background:yellow");    
    

console.log in NodeJS

console.log is also used in NodeJS to print output in terminal.

console.warn

console.warn is used to print warnings in browser console.

    console.warn('its a warning');
            

console.error

  console.error('error found');
          

console.table

console.table is used to display Arrays and Objects data in console. The argument in table method is either a Array or Object.

Print object in console.table

    console.table({name:"avi",id:212})

Print array in console.table

    console.table(["abc","pqr","xyz"])

Print array of object in console.table

    console.table([{name:"avi",id:212},{name:"isha",id:200}])

console.time

console.time() is used to measure time ( in ms) taken by task to run.

console.time([label]) starts a new timer. To print code execution time since console.time([label]), use console.timeEnd([label]) method.

    console.time('timer');
    alert(2);
    console.timeEnd('timer');

Check Loop timer

console.time('timer');
for(var i=1; i<=1000; i++){
    document.querySelector('select.num').innerHTML+="<option>"+ i +"</option>";
}
console.timeEnd('timer');

console.clear

console.clear method is used to clear browser console. This will remove all previously printed console messages and print a messages "Console was cleared" in some browsers.

    console.clear();

console.assert

console.assert method is used to print errors if assertion is false. If it is true, it does nothing.

    console.assert(1===true);
                

    console.assert(1==true);