Type Narrowing for hasAttribute and getAttribute

[TheColorRed]

TypeScript Version: 2.7.1 and 2.8.0-insiders.20180127

Type guard should catch hasAttribute so that getAttribute doesn't return null.

In the following example, TS still says getAttribute could return null, however we previously checked for the attribute on the previous line.

let element = document.querySelector('#element')

if(element.hasAttribute('data-title')) {
  let el = element.getAttribute('data-title') // still returns "string | null"
}

Expected Result:

element.getAttribute('data-title') Return type should be string

Actual Result:

element.getAttribute('data-title') Return type is string | null

[weswigham]

I believe this would be an (very complex, but possible) override of hasAttribute and getAttribute Element signatures over at the lib generator - Microsoft/TSJS-lib-generator.

[mhegazy]

Duplicate of #18781 and #13086. I do nto think we want to add such overloads to the library, but you are welcomed to add them. @DanielRosenwasser has a sample how

[TheColorRed]

@mhegazy Isn't has for Set and Map? Isn't that different than hasAttribute?

[weswigham]

It's the same kinda situation; the types would look identical.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.