Changing variable by html button

<div id="game">
    <button onClick="choose('rock')">Rock</button>
    <button onClick="choose('paper')">Paper</button>
    <button onClick="choose('scissors')">Scissors</button>
    <div id="result"></div>
    <br>
    <br>
    <button onClick="test()">DEBUG</button>
</div>

and

var user;
function choose(choice){
    user = choice;
}

function test(click){
    alert("You chose " + user);
}                         

var is used for declaring a variable. You don't need to declare user variable again in user function. You just need to assign a value to declared one.

var user; //declaration
function user(choice) {
    user = choice; //assignment
}

One problem:

var user = "none";
function user(choice){
    var user = choice;
}

One variable of user is hiding the other variable of user.

And having a function and variable with the same name is a BAD idea.