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Hey, a learner myself here so take what I say with a grain of salt.
The way I understand it is that the serve() and kick() methods of their respective types return no value but run an action.
As they are just types they represent the object they are annotated to and neither return a value nor perform an action themselves but instruct the method to expect no return value.
Meaning, that the sport argument needs to be pre-defined before it is run through the play() function.
As this is just given as an example in the exercise the code above does nothing functionally. For it to work you’d need to define a sport object (provide type of either Tennis or Soccer, or at least infer it) and run the function with said sport argument.
I.E.:
type Tennis = {
serve: () => void;
}
type Soccer = {
kick: () => void;
}
function play(sport: Tennis | Soccer) {
if ('serve' in sport) {
return sport.serve();
}
if ('kick' in sport) {
return sport. Kick();
}
}
const football: Soccer = {
kick() {
// action somehow related to kicking goes here i.e. a web page creates a <div>Ball Kicked!/div>
const kickDiv = document.createElement('div');
div.innerText = "Ball Kicked!";
parentElement.appendChild(kickDiv);
}
}
const tennis: Tennis = {
serve() {
// returns a value like the serve velocity
return Math.floor(Math.random*100);
}
}
// Then the following is correct:
play(football); // runs
play(tennis); // typeError as tennis.serve returns a value
}
Someone with more experience, please, do correct me if I’m wrong.
P.S. My answer comes 2 years later, but I had the same question and tried looking here but had to go through the TS docs to understand it. Now at least there is something go on
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