How to properly redefine a cell?

[btheado]

I'm using this code to clear notebook-kit variables between redefining a cell:

const clearCellVariables = cell => {
  cell.variables.forEach(variable => variable.delete());
  cell.variables = [];
};

However, when the cell value changes from an javascript expression to another javascript expression, the output shows the string "undefined". When changing from an expression to a statement or vice versa it displays the proper value.

I have an example notebook (using embedded notebook-kit library) with the full code illustrating the problem at:

https://new.observablehq.com/@btheado/notebook-kit-clear-variables-test

Am I doing something wrong?

[btheado]

It seems to be at least partially a timing issue. If I add a one second delay between clearing and define like this:

const code = code_samples[changeCell % code_samples.length];
display(code);
debugger;
clearCellVariables(cell);
await Promises.delay(1000);
nr.define(cell, todef(transpileJavaScript(code), 1));

then eventually the correct value comes. When transitioning to either one of the 2nd two pieces of code, the "undefined" string appears in the output and gets replaced by the desired value after one second.

[mbostock]

This is a bit of a hack and we should probably fix it internally, but here’s what we currently use to dispose of a variable:

const noobserver: Observer = {};

function dispose(variable: Variable): void {
  variable._observer &&= noobserver; // don’t render undefined
  variable.delete();
}

You could use it like so:

cell.variables.forEach(dispose);
cell.variables = [];

We probably should offer a variable.dispose() method that does this directly (where unlike variable.delete(), you’re not intended to use the variable after it has been disposed).

Possibly we should instead do something like runtime.dispose() does:

function dispose(variable: Variable): void {
  variable._invalidate();
  variable._version = NaN;
  variable.delete();
}

[btheado]

This is a bit of a hack and we should probably fix it internally, but here’s what we currently use to dispose of a variable:

Thanks! I applied that change to my notebook and republished it. It works as I expect now.