Version 1.9.0 introduced a new issue in exceljs/index.d.ts.

ERROR in node_modules/exceljs/index.d.ts(787,17): error TS2371: A parameter initializer is only allowed in a function or constructor implementation.

In an Angular (7) project, install exceljs 1.9.0 and add the following to tsconfig.json

"paths": {
  "exceljs": ["../node_modules/exceljs/dist/exceljs.min"]
}

use ng serve and see TS2371

Downgrading to 1.8.0 solves the problem