Setting default value for TypeScript object passed as argument

function sayName(params: {firstName: string; lastName?: string}) {
    params.lastName = params.lastName || 'smith';  // <<-- any better alternative to this?
    var name = params.firstName + params.lastName
    alert(name);
}

sayName({firstName: 'bob'});

I had imagined something like this might work:

function sayName(params: {firstName: string; lastName: string = 'smith'}) {

Obviously if these were plain arguments you could do it with:

function sayName(firstName: string, lastName="smith") {
    var name = firstName + lastName;
    alert(name);
}

sayName('bob');

And in coffeescript you have access to the conditional existence operator so can do:

param.lastName ?= 'smith'

Which compiles to the javascript:

if (param.lastName == null) {
    param.lastName="smith";
}