Javascript nested objects to JSON: how to filter attributes

Question

I have a hierarchy of JS Objects, like this:

function Obj1(){
    this.att1;
    this.Obj2Array;
}
function Obj2(){
    this.att1;
    this.att2;  
}

where Obj1 has a reference to an array of Obj2. As you can see Obj1 and Obj2 can have similar attribute names. no guarantee for uniqueness.

I want to get the JSON of Obj1, and I want to exclude some attributes of Obj2. I know that stringify receives a replacer function or array, And I have tried it but there is the following problem:

When I use a replacer function, how can I differentiate between attributes in Obj1 and Obj2, even if they have the same name? my final goal is to have a behavior like Java toString, where each Object gets to make a decision about its attributes:

Obj1.prototype.stringify = function (key, val){
    // if own attribute, return val;
    // else it is an Obj2 attribute, return Obj2.prototype.stringify(key, val)
}
Obj2.prototype.stringify = function (key, val){
    if (key == "att1"){
        return "";
    } else if (key == "att2"){
        return val;
    }
}

I suppose I am missing a better solution.

Makan · Accepted Answer · 2015-12-20 12:28:57Z

2

A good solution is using 'toJSON' function!

Just as Java's printing operation that calls toString, In Javascript, the JSON.stringify function calls the object's 'toJSON' function. A user defined toJSON function changes the behavior and you can pick each object's attributes. It goes like this:

Obj1.prototype.toJSON = function (){
    return {
        att1: this.att1,
        obj2array: this.Obj2Array
    };
}

Obj2.prototype.toJSON = function (){
    return {
        att2: this.att2
    };
}

for using it:

var o1 = new Obj1;
// assign anything you want to o1...
JSON.stringify(o1);

answered Dec 20, 2015 at 12:28

Makan

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Comments

Jose Mato · Accepted Answer · 2015-12-19 09:52:25Z

1

Reading your code i think that you want keep attributes from Obj1 and only get non existent attributes for Obj2. You can do that with assign method in the next way:

var Obj1 = {
    attr1: 'foo'
};

var Obj2 = {
    attr1: 'foo2',
    attr2: 'bar'
};
// now, c has attr1: 'foo' (from Obj1) and attr2: 'bar' (from Obj2)    
var c = Object.assign({}, Obj2, Obj1);

// and finally call json stringify with new object
JSON.stringify(c);

With Object.assign you can clone or merge objects: https://developer.mozilla.org/es/docs/Web/JavaScript/Referencia/Objetos_globales/Object/assign

answered Dec 19, 2015 at 9:52

Jose Mato

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1 Comment

Makan Over a year ago

Thanks for the answer. I provided an example, and although it works in this case, its not a general approach. while as you can see I have defined constructors and prototypes and I am in need of a solution similar to java 'toString'. which would give us much more power. I am not looking for a one line workaround. (Would you mind referencing to an English document? I googled it, but its nice to have the English version straightforward here.)

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