If you're like me and you use console.log as a debugging method by printing things to the console, the least we can do is to make the outputs prettier. What if our logs could be printed in different colors and font sizes, like in the print screen bellow?

In order to style (format) our logs we must use so called format specifiers. They contain the % symbol, followed by a letter that specifies what kind of formatting we want to the apply to our output.

They look like this:

%s - Formats the value as a string
%i or %d - Formats the value as an integer
%f - Formats the value as a floating point value
%o - Formats the value as an expandable DOM element. As seen in the Elements panel 
%O - Formats the value as an expandable JavaScript object
%c - Applies CSS style rules to the output string as specified by the second parameter

Now, let's write some examples. You can follow along by copy-pasting them in your Chrome console and see the result.

Example: print a blue string (apply CSS style)

console.log("%cThis will be formatted with blue text", "color: blue"); 
// outputs: This will be formatted with blue text [in blue color]

We can add as many styles as we wish

console.log("%cTry me on!", "color: #FFFFFF; font-size: 45px; background: #333333; text-shadow: #FFF 0px 0px 5px, #FFF 0px 0px 10px, #FFF 0px 0px 15px, #FF2D95 0px 0px 20px, #FF2D95 0px 0px 30px, #FF2D95 0px 0px 40px, #FF2D95 0px 0px 50px, #FF2D95 0px 0px 75px;")
// outputs: a Vegas style super shiny string

Styling is not the only thing we can manipulate in the console. We can convert data types (e.g. a number to a string) or output them (e.g. print objects or floats). Just check the examples below.

Example: print a string (convert a number to a string)

console.log("This will be formatted as a string - %s ", 8999); 
// outputs: This will be formatted as an integer - 8999 

Example: print a string (convert an array to a string)

console.log("This will be formatted as a string - %s ", [78, 89, 1024, 47]); 
// outputs: This will be formatted as a string - Array(4) 

You can't actually interact with the output in the console, so you can't see the content of the array since it's just a string.

Example: print an object

console.log('This is an object %o', { obj: { prop1: 'Hello', prop2: "World" }})
// outputs: this is an object {obj: {…}}

You can interact with the output in the console, expand the object and see its properties

Example: print an integer or a float

console.log('Integer: %d, Floating point: %.1f', 12, 7.3)
// output: Integer: 12, Floating point: 7.3

LATER EDIT - grouping specifiers

If we want to use multiple format specifiers at the same time, we can do it like so:

console.log("%cThis will be formatted with blue text This will be formatted as a string - %s", "color: blue", 8999)
// outputs: This will be formatted with blue text This will be formatted as a string - 8999 [all in blue color]

What we do is basically provide all the format specifiers in the first string and then provide the arguments, one by one (in quotes or not, depending on what you're trying to achieve - CSS rules and strings need quotes for example, numbers or arrays don't).