Did you know that you can pass a function as the second parameter to JSON.parse()? Doing this causes every key/value pair to be passed to this function for further processing. You can read all about how JSON.parse() and the reviver function works here on MDN’s site.

Example Reviver

We can actually leverage this functionality to turn all date strings into proper Date objects in JavaScript. The following is the JavaScript function that we can use as our “reviver” function:

/**
 * The reviver function that can be passed as the second argument to
 * `JSON.parse()` so that all date strings will be parsed as proper dates.
 */
 function dateReviver(key, value) {
  // If the value is a string and if it roughly looks like it could be a
  // JSON-style date string go ahead and try to parse it as a Date object.
  if ('string' === typeof value && /^\d{4}-[01]\d-[0-3]\dT[012]\d(?::[0-6]\d){2}\.\d{3}Z$/.test(value)) {
    var date = new Date(value);
    // If the date is valid then go ahead and return the date object.
    if (+date === +date) {
      return date;
    }
  }
  // If a date was not returned, return the value that was passed in.
  return value;
}

How To Use It

In order to use it, it is really as simple as passing it as the second argument to JSON.parse(). Here is an example showing how to use the reviver function:

var dtNow = new Date();
var jsonDtNow = JSON.stringify(dtNow);
var parsedDt = JSON.parse(jsonDtNow, dateReviver);
console.log(dtNow.getTime() === parsedDt.getTime());

If you run the dateReviver along with the above code you will see that the date/time values of dtNow and parsedDt are exactly the same. This same concept works for nested structures as shown below:

var person = {
  name: "John Smith",
  dateOfBirth: new Date(1950, 1, 1)
};
var jsonPerson = JSON.stringify(person);
var parsedPerson = JSON.parse(jsonPerson, dateReviver);
console.log(person.dateOfBirth.getTime() === parsedPerson.dateOfBirth.getTime());

That’s pretty much all I have to say about JSON.parse() and the reviver function. Let me know if you have any questions or comments below and as always happy coding! šŸ˜Ž

Categories: BlogJavaScript

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