javascript - JS setInterval executes only once

Question

I have the following JS functions:

function checkIfGameAlreadyStarted(){
    $.get("IsGameAlreadyStarted",null,function(gameAlreadyStarted){
        if (gameAlreadyStarted == "true"){
            window.location = "index.jsp?content=game";       
        } else{
            alert("bla");
        }  
    });
}

function joinGame(playerForm){
    $.get("GenerateClientID",null,function(clientID){         
        $.get("JoinGame",{
            "NAME" : playerForm.elements[0].value,
            "ID" : clientID
        }
        ,function(gameParam){

            $("#waitingContainer").append("You have joined the game!<br>Waiting for game creator to start game..");
            setInterval(checkIfGameAlreadyStarted(), 1000);    

        });
    });
}

Why does setInterval executes checkIfGameAlreadyStarted only once, and not every second?

Answer 1

You are passing the result of executing the function instead of the function itself. Since the result of the function is undefined, you are executing checkIfGameAlreadyStarted and then passing undefined to setInterval which doesn't do anything.

Instead of this:

setInterval(checkIfGameAlreadyStarted(), 1000);

Your statement should be this:

setInterval(checkIfGameAlreadyStarted, 1000);

without the parentheses at the end of the function name.

When you pass checkIfGameAlreadyStarted() that calls the function immediately and gets it's return value. When you pass checkIfGameAlreadyStarted that passes a reference to the function so setInterval can call it later (which is what you want).