Can I conditionally pass a property into an object?

You could use that syntax if you wanted to choose between different methods dynamically. But there's no "do nothing" method that you can substitute in place of y when the condition is false.

Use an if statement instead.

let temp = myFunction.x();
if (isTrue) {
    temp = temp.y();
}
temp.x()

Here's another solution that is more like what you originally imagined. Barmar's solution will work just fine, but I thought I'd show a different approach more like you imagined:

obj
  .x()
  [doIt ? "y" : "noop"]()
  .x();

In this scheme, doIt is any boolean or boolean expression and .noop() is a "do nothing" method on the object.

And, here's a working example you can run in a snippet:

const obj = {
    x() {
        console.log("executing x() method");
        return this;
    },
    y() {
        console.log("executing y() method");
        return this;
    },
    noop() {
        console.log("executing noop() method");
        return this;
    }
}

// try both values of the boolean
for (let doIt of [true, false]) {
    console.log(`\nresults for doIt = ${doIt}`);
    obj
        .x()
        [doIt ? "y" : "noop"]()
        .x();
}