JSON.stringify in Node.js

Question

Why when I use JSON.stringify in node.js on data from database request I get objects with the data itself, and if I use console.log I get a lot of additional parameters like dataValues, _previousDataValues, _modelOptions.... I use sequelize.js

console.log(JSON.stringify(data,'',2))

{
  id:1
}

console.log(data)

Kindergarten_data {
  dataValues:
    { id: 1}
 _changed: {},
  _modelOptions:
   { timestamps: true,
     validate: {},
     freezeTableName: true,
     underscored: false,
     underscoredAll: false,
     paranoid: false,
     rejectOnEmpty: false,
     whereCollection: null,
     schema: null,
     schemaDelimiter: '',
     defaultScope: {},
     scopes: [],
     indexes: [],...

And what's the problem? You're using sequelize, it returns you models, e.g. objects with data, methods, hooks etc. — Styx
– Styx, Commented Apr 4, 2019 at 10:36
I want to know why JSON.stringify shows only data values from database . Why not full object? Why JSON.stringify ignore _previousDataValues, _modelOptions.... — Paweł Baca
– Paweł Baca, Commented Apr 4, 2019 at 10:39

tevemadar · Accepted Answer · 2019-04-04 11:34:32Z

JSON.stringify() has a protocol which it follows:

JSON.stringify() converts a value to JSON notation representing it:

If the value has a toJSON() method, it's responsible to define what data will be serialized.

Boolean, Number, and String objects are converted to the corresponding primitive values during stringification, in accord with the traditional conversion semantics.

[...]

It is pretty obvious that your stringified stuff simply does not follow what you see with console.log(), since you would have to see { dataValues: {id: 1} ..., but you see {id: 1} instead. So it is not about some mystical "JSON-representability" other answers may suggest, but sequelize.js simply provides toJSON() method, and thus it JSON-exports only your data, not the entire infrastructure.

The method is here: https://github.com/sequelize/sequelize/blob/master/lib/model.js#L4226

toJSON() {
  return _.cloneDeep(
    this.get({
      plain: true
    })
  );
}

The get() it refers is here: https://github.com/sequelize/sequelize/blob/master/lib/model.js#L3313

get(key, options) {
  if (options === undefined && typeof key === 'object') {
    options = key;
    key = undefined;
  }

  [...]

  if (key) {
    [...]
  }

  return this.dataValues;
}

cloneDeep() just makes a copy of the thing, it does not matter here.
What matters is the return this.dataValues; above. That is the line which strips the object you see in the console, and results in serializing only its dataValues field.

Small-scale demo:

var data1={
  dataValues:{id:1},
  somethingElse:[1,2,3],
  _stuff:{a:'b'}
};

var data2={
  dataValues:{id:1},
  somethingElse:[1,2,3],
  _stuff:{a:'b'},
  toJSON:function(){return this.dataValues;}
};

console.log("data1:",JSON.stringify(data1));
console.log("data2:",JSON.stringify(data2));

PrivateOmega · Accepted Answer · 2019-04-04 10:43:07Z

-1

JSON.stringify() returns JSON-safe string representation of its input(eg JS object). It will strip off all properties/fields which doesn't have an equivalent JSON representation. Whereas console.log() outputs the JS object as it is to the console.

answered Apr 4, 2019 at 10:43

PrivateOmega

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1 Comment

Styx Over a year ago

You answer is incomplete. For example, _modelOptions (from example) does have JSON representation. See answer from @tevemodar

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