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Previously we only had abstract vals and vars, for which `typer` does not create a private field.","created_at":"2016-11-30T21:37:24Z","updated_at":"2016-11-30T21:37:34Z","html_url":"https://github.com/scala-js/scala-js/pull/2662#discussion_r90333952","pull_request_url":"https://api.github.com/repos/scala-js/scala-js/pulls/2662","_links":{"self":{"href":"https://api.github.com/repos/scala-js/scala-js/pulls/comments/90333952"},"html":{"href":"https://github.com/scala-js/scala-js/pull/2662#discussion_r90333952"},"pull_request":{"href":"https://api.github.com/repos/scala-js/scala-js/pulls/2662"}}},"pull_request":{"url":"https://api.github.com/repos/scala-js/scala-js/pulls/2662","id":95352858,"html_url":"https://github.com/scala-js/scala-js/pull/2662","diff_url":"https://github.com/scala-js/scala-js/pull/2662.diff","patch_url":"https://github.com/scala-js/scala-js/pull/2662.patch","issue_url":"https://api.github.com/repos/scala-js/scala-js/issues/2662","number":2662,"state":"open","locked":false,"title":"Add an annotation `@JSOptional` for optional members in JS types.","user":{"login":"sjrd","id":535934,"avatar_url":"https://avatars.githubusercontent.com/u/535934?v=3","gravatar_id":"","url":"https://api.github.com/users/sjrd","html_url":"https://github.com/sjrd","followers_url":"https://api.github.com/users/sjrd/followers","following_url":"https://api.github.com/users/sjrd/following{/other_user}","gists_url":"https://api.github.com/users/sjrd/gists{/gist_id}","starred_url":"https://api.github.com/users/sjrd/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/sjrd/subscriptions","organizations_url":"https://api.github.com/users/sjrd/orgs","repos_url":"https://api.github.com/users/sjrd/repos","events_url":"https://api.github.com/users/sjrd/events{/privacy}","received_events_url":"https://api.github.com/users/sjrd/received_events","type":"User","site_admin":false},"body":"In a JS type, and particularly in a Scala.js-defined JS type, vals,\r\nvars and defs without parentheses can be annotated with\r\n`@JSOptional`. In that case, they must be concrete, and their rhs\r\nmust be `js.undefined`. This is also valid in a Scala.js-defined JS\r\ntrait, and is the only way to define a concrete member in such a\r\ntrait.\r\n\r\nAn `@JSOptional` member does not create a field nor getter in the\r\nobject or class. Instead, it gets a value of `undefined` by virtue\r\nof JavaScript using `undefined` for missing fields. In case of a\r\n`var`, the field can be created later by explicit assignment. For\r\n`val`s and `def`s, it can be created in subclasses with an\r\n`override val` or `override def`.\r\n\r\nThis is particularly useful to model \"configuration objects\" as\r\nused in many JavaScript APIs. A Scala.js-defined JS trait with\r\n`JSOptional` fields can define a typed API for the configuration\r\nobject. It is then easy to create a configuration object with only\r\na few fields set by defining `override val`s. 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