How do I access properties of a javascript object if I don't know the names?

Question

Say you have a javascript object like this:

var data = { foo: 'bar', baz: 'quux' };

You can access the properties by the property name:

var foo = data.foo;
var baz = data["baz"];

But is it possible to get these values if you don't know the name of the properties? Does the unordered nature of these properties make it impossible to tell them apart?

In my case I'm thinking specifically of a situation where a function needs to accept a series of name-value pairs, but the names of the properties may change.

My thoughts on how to do this so far is to pass the names of the properties to the function along with the data, but this feels like a hack. I would prefer to do this with introspection if possible.

Answer 1

You can loop through keys like this:

for (var key in data) {
  console.log(key);
}

This logs "Name" and "Value".

If you have a more complex object type (not just a plain hash-like object, as in the original question), you'll want to only loop through keys that belong to the object itself, as opposed to keys on the object's prototype:

for (var key in data) {
  if (data.hasOwnProperty(key)) {
    console.log(key);
  }
}

As you noted, keys are not guaranteed to be in any particular order. Note how this differs from the following:

for each (var value in data) {
  console.log(value);
}

This example loops through values, so it would log Property Name and 0. N.B.: The for each syntax is mostly only supported in Firefox, but not in other browsers.

If your target browsers support ES5, or your site includes es5-shim.js (recommended), you can also use Object.keys:

var data = { Name: 'Property Name', Value: '0' };
console.log(Object.keys(data)); // => ["Name", "Value"]

and loop with Array.prototype.forEach:

Object.keys(data).forEach(function (key) {
  console.log(data[key]);
});
// => Logs "Property Name", 0

Answer 2

Old versions of JavaScript (< ES5) require using a for..in loop:

for (var key in data) {
  if (data.hasOwnProperty(key)) {
    // do something with key
  }
}

ES5 introduces Object.keys and Array#forEach which makes this a little easier:

var data = { foo: 'bar', baz: 'quux' };

Object.keys(data); // ['foo', 'baz']
Object.keys(data).map(function(key){ return data[key] }) // ['bar', 'quux']
Object.keys(data).forEach(function (key) {
  // do something with data[key]
});

ES2017 introduces Object.values and Object.entries.

Object.values(data) // ['bar', 'quux']
Object.entries(data) // [['foo', 'bar'], ['baz', 'quux']]

Answer 3

for(var property in data) {
    alert(property);
}

Answer 4

You often will want to examine the particular properties of an instance of an object, without all of it's shared prototype methods and properties:

 Obj.prototype.toString= function(){
        var A= [];
        for(var p in this){
            if(this.hasOwnProperty(p)){
                A[A.length]= p+'='+this[p];
            }
        }

    return A.join(', ');
}

Answer 5

function getDetailedObject(inputObject) {
    var detailedObject = {}, properties;

    do {
        properties = Object.getOwnPropertyNames( inputObject );
        for (var o in properties) {
            detailedObject[properties[o]] = inputObject[properties[o]];
        }
    } while ( inputObject = Object.getPrototypeOf( inputObject ) );

    return detailedObject;
}

This will get all properties and their values (inherited or own, enumerable or not) in a new object. original object is untouched. Now new object can be traversed using

var obj = { 'b': '4' }; //example object
var detailedObject = getDetailedObject(obj);
for(var o in detailedObject) {
    console.log('key: ' + o + '   value: ' + detailedObject[o]);
}

Answer 6

var obj = {
 a: [1, 3, 4],
 b: 2,
 c: ['hi', 'there']
 }
for(let r in obj){  //for in loop iterates all properties in an object
 console.log(r) ;  //print all properties in sequence
 console.log(obj[r]);//print all properties values
}

Answer 7

var fs   = require("fs");

fs.stat( process.argv[1], function( err, stats ){
if (err) {
     console.log( err.message ); 
     return;    
} else {
 console.log(JSON.stringify(stats));

/*  this is the answer here  */

    for (var key in Object.keys(stats)){
    var t = Object.keys( stats )[key];
    console.log( t + " value =: " + stats[t]  );
     }  

/*  to here, run in node */
   }
});

Answer 8

var attr, object_information='';

for(attr in object){

      //Get names and values of propertys with style (name : value)
      object_information += attr + ' : ' + object[attr] + '\n'; 

   }


alert(object_information); //Show all Object