Use array.push() to add an item to the end of the array.

var sample = new Array();
sample.push(new Object());

To do this n times use a for loop.

var n = 100;
var sample = new Array();
for (var i = 0; i < n; i++)
    sample.push(new Object());

Note that you can also substitute new Array() with [] and new Object() with {} so it becomes:

var n = 100;
var sample = [];
for (var i = 0; i < n; i++)
    sample.push({});

The following worked for me:

function Base() {
    this.n = 5;
    this.obj = [];
    for (var i = 0; i < this.n; i++) {
        this.obj.push(new Test());
    }
}

Where Test was:

var Test = function () {};

It seems like the real problem is that it never created this.obj because n was undefined. If you want to declare the array how you did before, try new Array(this.n).

EDIT

Would new Array(this.n) be faster than the this.obj=[] syntax since it's pre-allocated?

Interestingly enough, the answer is no.

Additionally, the accepted answer on this question has a great discussion of lots of other aspects of JavaScript array performance: What is the performance of Objects/Arrays in JavaScript? (specifically for Google V8)

I've updated my answer to use Array.push() because it is apparently much, much faster than doing Array[i] = new Obj() (shout out to Jason, who used it in his original answer).

You're defining the array correctly, but that's not how you iterate over an array in JavaScript. Try this:

function templateOptionList() {
    for(var i=0, l=templates.length; i<l; i++) {
        var t=templates[i];
        console.log(t.post_title);
    }
}

A nicer (albeit a little slower) way of doing this that only works in newer browsers would be to use Array.forEach:

function templateOptionList() {
    templates.forEach(function(t) {
        console.log(t.post_title);
    });
}

var Thing = function(params) {
  this.prop1 = params.prop1;
  this.prop2 = params.prop2;
};

var things = [
  new Thing({
    prop1: 'foo',
    prop2: 'bar'
  }),
  new Thing({
    prop1: 'foo',
    prop2: 'bar'
  }),
];