// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
function formatValue(value: string): [string, boolean] {
  const hasExpandString = /\%[^\%]*\%/i;
  return [value.replace(/^[\'\"].*[\'\"]$/, ''), hasExpandString.test(value)];
}

function formatOptions(options: string[]): [string, string, string, boolean] {
  let key, value, operator;
  switch (options.length) {
    case 2:
      [key, value] = options;
      return [key, '=', ...formatValue(value)]; 
      //                       ^
      // Type 'string | boolean' is not assignable to type 'string'.
      // Type 'false' is not assignable to type 'string'.
    default:
      throw new Error('argument error');
  }
}