How does node.js configurable logger work?

Question

I am reading a book called Node.js in Action by Nike Cantelon and stuck at a configurable logger implementation:

when using String.prototype.replace() we set a function(match, property) as a second parameter and I am clueless about what it does . Could anyone explain what function(match, property) does? The way code is written doesn't give me an insight into that..

function setup(format){
let regex = /:(\w+)/g;
return function logger(req, res, next){
      let str = format.replace(regex, (match, property) => {
          return req[property];
      });
      console.log(str);
      next();
  }
}

module.exports = setup;

Answer 1

The String.replace() function takes two arguments: searchvalue and newvalue . This means that you are searching for a particular pattern in your format string. When found, it is replaced by the value of req[property] . So the function is getting the property as it's argument, then uses it as a key for req object, getting the value back and replacing the found appearance in format string.