Difference between toJSON() and JSON.Stringify() Difference between toJSON() and JSON.Stringify() javascript javascript

Difference between toJSON() and JSON.Stringify()


From the fine manual:

toJSON behavior

If an object being stringified has a property named toJSON whose value is a function, then the toJSON method customizes JSON stringification behavior: instead of the object being serialized, the value returned by the toJSON method when called will be serialized.

This is why Backbone uses the toJSON method for serialization and given a model instance called m, you can say things like:

var string = JSON.stringify(m);

and get just the attributes out of m rather than a bunch of noise that your server won't care about.

That said, the main difference is that toJSON produces a value (a number, boolean, object, ...) that gets converted to a JSON string whereas JSON.stringify always produces a string.

The default Backbone toJSON is simply this (for models):

return _.clone(this.attributes);

so m.toJSON() gives you a shallow copy of the model's attributes. If there are arrays or objects as attribute values then you will end unexpected reference sharing. Note that Backbone.Model#clone also suffers from this problem.

If you want to safely clone a model's data then you could send it through JSON.stringify and then JSON.parse to get a deep copy:

var data         = JSON.parse(JSON.stringify(model_instance));var cloned_model = new M(data);

where model_instance is your instance of the Backbone model M.


  • JSON.stringify() - Any valid JSON representation value can be stringified.

    The JSON.stringify(..) utility will automatically omit undefined, function, and symbol values when it comes across them. If such a value is found in an array, that value is replaced by null (so that the array position information isn't altered). If found as a property of an object, that property will simply be excluded.

    JSON stringification has the special behavior that if an object value has a toJSON() method defined, this method will be called first to get a value to use for serialization.

  • toJSON() - to a valid JSON value suitable for stringification.

    One example, JSON.stringify() an object with circular reference in it, an error will be thrown. toJSON() can fix it as following.

    var o = { };var a = {    b: 32,    c: o};// circular referenceo.d = a;// JSON.stringify( a ); // an error caused by circular reference// define toJSON methoda.toJSON = function() {     return { b: this.b };};JSON.stringify( a ); // "{"b":32}"


I'm also reading Addy Osmani's Developing backbone.js application, and I have the same question. I figured out by trying his example (the todo list) in the console.

var Todo = Backbone.Model.extend({    defaults:{         title:"",         completed:false}});var todo1 = new Todo(); console.log(todo1.toJSON())//The console shows//Object {title: "finish your assignment", completed: false}console.log(JSON.stringify(todo1))//The console shows//{"title":"finish your assignment","completed":false}