json.stringify does not process object methods

Functions in javascript are more than just their code. They also have scope. Code can be stringified, but scope cannot.

JSON.stringify() will encode values that JSON supports. Objects with values that can be objects, arrays, strings, numbers and booleans. Anything else will be ignored or throw errors. Functions are not a supported entity in JSON. JSON handles pure data only, functions are not data, but behavior with more complex semantics.

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That said you can change how JSON.stringify() works. The second argument is a replacer function. So you could force the behavior you want by forcing the strinigification of functions:

var obj = {
  foo: function() {
    return "I'm a function!";
  }
};

var json = JSON.stringify(obj, function(key, value) {
  if (typeof value === 'function') {
    return value.toString();
  } else {
    return value;
  }
});

console.log(json);
// {"foo":"function () { return \"I'm a function!\" }"}

But when you read that back in you would have to eval the function string and set the result back to the object, because JSON does not support functions.

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All in all encoding functions in JSON can get pretty hairy. Are you sure you want to do this? There is probably a better way…

Perhaps you could instead save raw data, and pass that to a constructor from your JS loaded on the page. localStorage would only hold the data, but your code loaded onto the page would provide the methods to operate on that data.

// contrived example...

var MyClass = function(data) {
  this.firstName = data.firstName;
  this.lastName = data.lastName;
}

MyClass.prototype.getName() {
  return this.firstName + ' ' + this.lastName;
}

localStorage.peopleData = [{
  firstName: 'Bob',
  lastName:  'McDudeFace'
}];

var peopleData = localStorage.peopleData;

var bob = new MyClass(peopleData[0]);
bob.getName() // 'Bob McDudeFace'

We don’t need to save the getName() method to localStorage. We just need to feed that data into a constructor that will provide that method.