Optional arguments in JSDoc types may be moved to TypeScript types quickfix

TypeScript Version: 3.4.3

• 80004
• jsdoc quickfix optional

Code

With Closure Compiler, you mark a function parameter optional by adding an = after its type (reference):

/**
 * @param a {string} Base string.
 * @param b {string=} Optional string to add
 * @return {string}
 */
function foo(a, b) {
  if (b) return a + b;
  return a;
}

TypeScript offers a quickfix to convert these jsdoc annotations to TypeScript types:

Expected behavior:

The b parameter should be optional:

/**
 * @param a {string} Base string.
 * @param b {string=} Optional string to add
 * @return {string}
 */
function foo(a: string, b?: string): string {
  if (b) return a + b;
  return a;
}

Actual behavior:

The b parameter is required but has type string | optional:

/**
 * @param a {string} Base string.
 * @param b {string=} Optional string to add
 * @return {string}
 */
function foo(a: string, b: string | undefined): string {
  if (b) return a + b;
  return a;
}

This means that all calls of the function which don't specify b will be flagged as errors.

Playground Link: link, though quickfixes are not available there

Related Issues:

Start by reproducing the JSDoc-to-TypeScript types quickfix with the provided foo function and compare the generated parameter type with the expected optional parameter. Trace the quickfix entry point and add regression coverage showing that calls omitting b are accepted while the generated return type remains string.