I have a #TypeScript mapped type problem I don't know how to tackle:

I am looking for a way to turn this JS object: { a: true, b: false } into this
TS type: { a: string, b?: string }

Any pointers?

Playground link:
https://www.typescriptlang.org/play?ts=4.6.2#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-XyWDRA1uqk0+oqck3d3Up71KAASkLWqRupaB0h0AUZ02LVzTg4Kl+RQIAKB2C0-ZwbI5kpTjaOChM0GAwAQOQ1eDu-ugYoAQsK5wBZYhE7x1Ej6fSwGZzBZwdul2kkEa2sedM74IgC1vQLFEyhaU-zTp01W19NqiZeoBUvfMOArCsmygABIOC0AmAUAEJvwWKDuieDB1Fg+DEPbDh32JP8DhI9kKnvXIKxwuDGjuMhoB5eiAFECDuAgBQAA14SgfFEFcbAwRAWh2Uo6lyFQABIFA-EwkI4qjIDgswDVnaAAAlqWMMhjBQQT6MaZpoHwkhJAYAx9GoJoGiSbxllcTdoJUudM2IRlwgCBzM2CNz-Ac5T5ETZMjPdAouCLLAAMkKBlI2TIthEnAjgSvZOXY5sDVc1R1C0VSAEFbMQBh9EqQRjDtKVcjOHQ8AMIwZznQR8TjJwnkyAxoqg4hMTOTKhw0KqQm8JlsGi+r1MQfT6H9XdjAS6Amj6YxGhsRwXBiSBZCAA