What you are seeing there is the string substitution patterns that are built into console.log() or console.debug().

The pattern goes as I have presented below:

%s for a String

%d or %i for Number

%f for Floating points

%o for an Object

%j for an JSON

So essentially you are replacing the specifier with the values supplied as so:

var name = ‘Chris’;
console.log(‘Hi, my name is %s.’, name);
// Hi, my name is Chris.

console.debug(‘Hi, my name is %s.’, name);
// Hi, my name is Chris.

https://nodejs.org/api/util.html#util_util_format_format_args