What happens if I don’t pass a parameter in a Javascript function?

Nothing will happen- meaning you won’t get an error or a warning as passing the parameters in javascript is optional.
All the parameters that weren’t “supplied” will have the undefined value.

function foo(x, y, z){
    //...
}

foo(1);

Inside the foo function now:

function foo(x, y, z){
    x === 1
    y === undefined
    z === undefined
}

You can even pass more arguments, like:

foo(1,2,3,4,5,7); // Valid!

You can know the amounts of parameters supplied by arguments.length from inside the function.

function foo(x, y, z) {
    console.log('x value: ' + x);
    console.log('y value: ' + y);
    console.log('z value: ' + z);
    console.log('Arguments length: ' + arguments.length);
}
console.log('Zero parameters');
foo();
console.log('Four parameters');
foo(1, 2, 3, 4);

Example of useful function that handle any amount of parameters:

function max() {
    var maxValue = arguments[0];
    for (var i = 1; i < arguments.length; i++) {
        if (maxValue < arguments[i]) {
            maxValue = arguments[i];
        }
    }
    return maxValue;
}

alert(max(1, 5, 7, 2, 88, 32, 44));