How does the Safari Javascript console predict properties of nonexistent objects?

Question

Please note, I have not been able to reproduce this behavior in Chrome or Firefox, just Safari.

Let’s say I have a function with two possible randomly-selected return values: { foo: 'test' } and { bar: 'test' }.

function getObj() {
    console.log('This code has been executed.');
    if (Math.random() > 0.5) {
        return { foo: 'test' };
    } else {
        return { bar: 'test' };
    }
}

When I type an invocation of the function but do not press “enter”, Safari’s predictions can still make assumptions about the return value. Sometimes it sees one return value, sometimes it sees the other.

Sometimes one:



Sometimes the other:

Obviously, by simply typing the function invocation, I’m not calling the function as I understand it, and I do not see a console.log saying “This code has been executed.” How does Safari do this?