Add a Key Value Pair to an array of objects in javascript?

Along with a single value like

myarray[0].address = "your address";

You could even add a nested property on the fly as below:

myarray[0].address = { presentAddress: "my present address..." };

and get the value as myarray[0].address.presentAddress;.

You can simply add properties ("fields") on the fly.

Try

myarray[0].Address = "123 Some St.";

or

myarray[0]["Address"] = "123 Some St.";

var myarray = [];

myarray.push({
    "Name": 'Adam',
    "Age": 33
});

myarray.push({
    "Name": 'Emily',
    "Age": 32
});

myarray[0]["Address"] = "123 Some St.";

console.log( JSON.stringify( myarray, null, 2 ) );