worker.js

let sheet, errs, vals;
self.onmessage = ({ data }) => {
  [sheet, errs, vals] = [data, {}, {}];

  for (const coord in sheet) {
    // Four variable names pointing to the same coordinate: A1, a1, $A1, $a1
    ["", "$"].map(p =>
      [coord, coord.toLowerCase()].map(c => {
        const name = p + c;

        // Worker is reused across computations, so only define each variable once
        if ((Object.getOwnPropertyDescriptor(self, name) || {}).get) {
          return;
        }

        // Define self['A1'], w`hich is the same thing as the global variable A1
        Object.defineProperty(self, name, {
          get() {
            if (coord in vals) {
              return vals[coord];
            }
            vals[coord] = NaN;

            // Turn numeric strings into numbers, so =A1+C1 works when both are numbers
            let x = +sheet[coord];
            if (sheet[coord] !== x.toString()) {
              x = sheet[coord];
            }

            // Evaluate formula cells that begin with =
            try {
              vals[coord] = "=" === x[0] ? eval.call(null, x.slice(1)) : x;
            } catch (e) {
              const match = /\$?[A-Za-z]+[1-9][0-9]*\b/.exec(e);
              if (match && !(match[0] in self)) {
                // The formula refers to a uninitialized cell; set it to 0 and retry
                self[match[0]] = 0;
                delete vals[coord];
                return self[coord];
              }
              // Otherwise, stringify the caught exception in the errs object
              errs[coord] = e.toString();
            }

            // Turn vals[coord] into a string if it's not a number or boolean
            switch (typeof vals[coord]) {
              case "function":
              case "object":
                vals[coord] += "";
            }
            return vals[coord];
          }
        });
      })
    );
  }

  // For each coordinate in the sheet, call the property getter defined above
  for (const coord in sheet) {
    self[coord];
  }
  postMessage([errs, vals]);
};