Iterating over properties requires this additional hasOwnProperty check:

for (var prop in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, prop)) {
        // do stuff
    }
}

It's necessary because an object's prototype contains additional properties for the object which are technically part of the object. These additional properties are inherited from the base object class, but are still properties of obj.

hasOwnProperty simply checks to see if this is a property specific to this class, and not one inherited from the base class.

It's also possible to call hasOwnProperty through the object itself:

if (obj.hasOwnProperty(prop)) {
    // do stuff
}

But this will fail if the object has an unrelated field with the same name:

var obj = { foo: 42, hasOwnProperty: 'lol' };
obj.hasOwnProperty('foo');  // TypeError: hasOwnProperty is not a function

That's why it's safer to call it through Object.prototype instead:

var obj = { foo: 42, hasOwnProperty: 'lol' };
Object.prototype.hasOwnProperty.call(obj, 'foo');  // true

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.

Here is the snippet:

var p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (var key in p) {
    if (p.hasOwnProperty(key)) {
        console.log(key + " -> " + p[key]);
    }
}

For-of with Object.keys() alternative:

var p = {
    0: "value1",
    "b": "value2",
    key: "value3"
};

for (var key of Object.keys(p)) {
    console.log(key + " -> " + p[key])
}

Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties

Using the new Object.entries() method:

Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.

const p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (const [key, value] of Object.entries(p)) {
  console.log(`{value}`);
}

For iterating on keys of Arrays, Strings, or Objects, use for .. in :

for (let key in yourobject) {
  console.log(key, yourobject[key]);
}

With ES6, if you need both keys and values simultaneously, do

for (let [key, value] of Object.entries(yourobject)) {
    console.log(key, value);
}

To avoid logging inherited properties, check with hasOwnProperty :

for (let key in yourobject) {
   if (yourobject.hasOwnProperty(key)) {
      console.log(key, yourobject[key]);
   }
}

You don't need to check hasOwnProperty when iterating on keys if you're using a simple object (for example one you made yourself with {}).

This MDN documentation explains more generally how to deal with objects and their properties.

If you want to do it "in chunks", the best is to extract the keys in an array. As the order isn't guaranteed, this is the proper way. In modern browsers, you can use

let keys = Object.keys(yourobject);

To be more compatible, you'd better do this :

 let keys = [];
 for (let key in yourobject) {      
     if (yourobject.hasOwnProperty(key)) keys.push(key);
 }

Then you can iterate on your properties by index: yourobject[keys[i]] :

for (let i=300; i < keys.length && i < 600; i++) { 
   console.log(keys[i], yourobject[keys[i]]);
}

var obj = {
    'foo': 1,
    'bar': 2
};

for (var key in obj) {
    console.log(obj[key]);
}

Or with jQuery:

$.each(obj, function(key, value) {
    console.log(this, value, obj[key]);
});