TypeScript Expressivity

## Index 1. Generalities 2. Everyday types 3. Common patterns 4. Advance typing 5. Limits of expressivity 6. Resources

## Generalities

- Powerful inference leverages **intentionality** - [Type compatibility](https://www.typescriptlang.org/docs/handbook/type-compatibility.html) is structural subtyping - Embraces JavaScript common patterns - Assumes **you** know more - Has no effect on runtime

## Everyday types

- Common types - `unknown` vs `any` - Optional keys, `null` and `undefined` - Type aliases [Try it out](https://www.typescriptlang.org/play?#code/PTAEFEDVwJQTQCIEE6gCpwArgMoCgRQBeE0ssvAsAYQHsBbe2gO1ABcBPABwFMBnKqAC0I0WJGVCAIjQALAJZ9Qi0AENQfNgCd5zAOYAadgqUr1zAK70ARjy0A6R1LwAbHm1AKAYloYA5OAAuDW1dPWJQAHJZWhd1ADNfelAAgEJI13c1LR0ANzC4HlUtYMsbOwiAJgAGGsyPAHdVPiQAcVpaABNC4uDrDrdVViJ2LQseetBOvLCAGXkAYx5mPh4kNjYeei42PmCEVU2AbQBdCKO8UFBmHgbQA82ACkiAKQtWSoBGIxqvyIBKAyXa63e6HHjPJAWcKfADMP1qnwBQKuNzuDwhkQA8gsPAA2BF-f54E6TGL0Hh0WhaTq6cF7UAAb1ALhYenkbAsnR4pSsti0RjibA5XJ51z5FQAvhFGcDWfoRdzgkIAOyw+wqvG1FEsw6KsUAFk+9mqsLxAFY8JLJGApEhQPF3rj5CxjIc1AsljslOpNDp9PZnI7mM7XXotDx3I9w5HhfoGX6wv8mcCFiw+LEePZWXpoxH3GE+MTrYI7Q6ncLXWxZO6I5ytCs1OLyg4gxWXawhnwGnY0LR5vEeABVZjyXJ2VZIZidcDjrQcathR7-XktlNXOsWBugA2VK020AyWQ8DQ8NPT0BcYqqCmbLTKJS0HYd1QuQN4YOh1hbLg1vjyAAvHhOlafM2DzWNC2CRN9CMH8-0UWYeHHFwAH5giGDhk1lK54OaRDkJ4FwIjw-8+CQlDQFQ1DQGqVN00zbNaFzGMC3jUAAGoonSewI14Q5HlIgiUP+YsDykK9OiQ+JwOTehVA4dhVAAaxPX1Qn0UBqSbMp+XfT9K1YSTpPAjNbwUfRoI0wxL1UTpaUs5t+VAAAfEJ-T0Fd3LCddlHiUBHk4XhaH8ySHPCUgol0uwAV8jd3C3VhIiiXieH48CwqTTiNAYdwLL0YFrXi+sjLs8LsrMvKwn3UtqCGUAtg5LSbi0-zq34E9COYXYok0YpNk6SIjEiTQn14QbnCCk9MDiDhZ2WDwRhGth+uA5K3OWsa1rwXQ73iVQllAGaFLscB6A5O9fJYQTx264Jjrm262CMeJmHQ0AvHbFgvNyWh5E6GrCAAA3eZTmFoBpmCB0BciUIHMKBwRxGRlHKDcDxVD8CUSicioRk+SZKsXRzMMmcGAEluVUYJQfByHKCJ-KIkx7G8EZnyRgpqm8BZtcRnZ-QeaxvnrloSminEuQTwMjsPS9HrCPnYm9H0r7WBzR4Bb0DDmCw3zzwzNwmNzLWxNLcmphYSIPDBiHQAaGsPBURclCvLQb3cOxVZDQyHU13LlZp5hbch7D6JWRiNdNwGwCxZ8WFfUBVI4PgjCBywXBcaGhk6UAQenHh4l0YDEcIFHy4ryvREl49bI4Vk7NABZ6rTCl7Y5GILA8dqHXkIjc8iCkWxwXQlkiSbuBPagLFGiktEwBSG9zkYcOUTpV35HVmA9qyPJ1If+RHkMeHejF93RpuZ7YXKcen2e7AX+vaEblfgX+4Jqi3neohwV9clUDI1oL6rFHNSDkikRgYnsHTZcwhL73wcAfOwR8lg1xPP-Fw4wm71VsKAd43Ii43E6BPXgR1fCdAsLiGUb9164wFMCbeFJd5hB1NyPgCwdDx2YMwzSbl8GF2LgDIBWRR4MC4OjHgmByGULYPdaRVDX5XHfrRL+TCohIHkgBV04AFgxEAZMLg8jZFkK6DI6hSjaHfAYd-SI7RmJuFAAACVykNYEbCOHyC4byTO58shDFoO1eeRi5GmIUb5ZRVjUQ2KQFwcRJ5nEUkwF0VxVx3GcMMkHAhgiY6HiljDV8WCVBPkMq+EhJ4ABC7g7xSNCYtcJtDor0KiWomChg3H8A8V48UPjhEY0qRsB+wTQD9OqUY8xa9giROuDYxxxR5KsAANLFE6K6cmzBfqqRSVMDp6SOzeJcDktAk81AuHkM0fgSMq7iQtk0bq7BaCgD4hGEBGNQCVAQJeP63V3wXyWN1OwwQjiNKMI0s4Iwjif1oqSUsABZRShjaAACszxOx9JnNQ-Qu6fN2qnUANyhgeHoIpCGrApoOm0u1eg74yVJN2u884wK6Gkgvu7acDBaV3SOl8tg9LwUwsOLIXiOcGDLiMPy6sQq2X0GXKSIAA) in the TypeScript playground!

## Common patterns

- [OOP goodies](https://www.typescriptlang.org/docs/handbook/2/classes.html) - [Generics](https://www.typescriptlang.org/docs/handbook/2/generics.html) - [Utility types](https://www.typescriptlang.org/docs/handbook/utility-types.html) [Try it out](https://www.typescriptlang.org/play?#code/PTAEGEHkFlsg5UAFAggFTQUQErwMoBQIoAvGeRZQUWJJEqAOYD2zAJgJYCmAzjaAFohwkQOrEARHmYBbLgBcAFhwB2jUAGMAhitAcZABwA2XOSvmglXUBIAyOtjK0AnANYS95rs4BmWjVwAdBIEqvLefgGg9iqOLq6gAN4EoKBGzGoc8gCubFwAXKAq2TIARt4paVryWbkFRSXlzgQAvtQaRlo8PBBZAJ56hiZm8j0xcW5JlRoZPPLO2RryzM4AFJWpBtmlRhwaRVpyhXPOqowANBugWzt7aRmMtXmFxWXel6mb27v7nTU5zwab2aqQAlFNUm02iYLPIXIwFOBWM5OCpqrxSEUuAB3XryPqrADkAHVlOFkJ1VDxCedBAB2ADMgTpADYAEwATlpABYAIyBAAMDO5AtB4jAEhQumYpQAVlwlnoelpNLIDMweFlrMwfDZwOkeFw2HhFFoDFwPA49cx0s4jSazRbgqEvL5-NZ9Rr7abzRDQC4uFpVqCXo13pUTGolMHQ8DWtQwhF3RAbStvY6-TNbcd5md4wQYaAdtkuABVFRZABCzAAHoVPYbjT7rAAyFO29O+kh+gNB8GJUB2nLOXS80AtD5pLhRxTBpKDhTZEegbnjydZlaFQnFriE-OSFCaTrdUA+FaqjtsIu1jH6Yymac1NSWRRezzhN0BHjOjpdHpIy9qxrQZ7xGf8DU7LhaQAtMmwzZJUgAfU1QFXiaSpEI3Zwc1ONRqFSGYVBORZljWLCcLOWkUPqNDvExXl+yuJQOG-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-XJ1X1bNzXGddm2w8KNkncjl23etz55zj0AGUT82tddq3J09oSD2lOUFVhU0LFMLJEtkaweHNDQOB8O4uEu8w0cO48egux9MBkLIP3xzBQC4drpzYHoTjOEnFcQ3Y5n+ob53ZhJVFATBOYAfkKAAxcLIoyFP8NVIiHc2tZqs+Ay55ukKegV4yhNSDJVjRI519pefgu8Xf9-ilRL6pGvl-Re7M35cG5o1HiSkQF3zAYcCBoBN6bzztxRSN8F5wPAVvQIWweCzlgd4MUkJj5Vw6uAwomBaSBBoQYLQfR0inUKLLLmgCT5zECpg7w99rIwNvtw+BchubINQWnQhzhvxnmcJgfwBD+F2RICTcRqwaG4PoYwtgoJiH2SEhjawSBOh9G8D3Sa3ZCTGHoYSUAAAfQ2cxmAGD3L+E8Bj6F0VHsFCe6827yD7gPbw+NXFGOkT4mex8LEEjYQZMhRIIk9TToRHgNogjpEYESIJZwijMGxKo+J9lKj2IMKJNB34YmEkKXkmqRFkmBFSUSPAywDCZJUNk3J2ioTtFqtcVy7kgl3TFAYQIL9zGGOenORRSQWgDKGSobyvA5pjPBBMxIUywalhqLsfElg+jmiBmAEGBywYBTtKdDIRgBglW4LiHUL5rCQwtJ3P8oA-YAEkiIN1ImzP0iEWmAgDnjNmx9Eln1Iq-PGkscYAuJlE6BPy8auTxoJY+zVbYlhjKAbAgZpoqHOYTZmUKOaEjDoSTmYS07iWXNfX5QQw5IsqCi02OdZkhgxVis5fQ8VSwJezbaztpwkrJZ8ClklYXUsCIyqOSKhKFipB8883Y3K4lee8su5kfZ+zFIWCVDNMSytVc4cVfKVCHSNbg0Ws49Z2xXNnKOltxygiAA) in the TypeScript playground!

## Advance typing

- [Template literal types](https://www.typescriptlang.org/docs/handbook/2/template-literal-types.html) - [Mapped types](https://www.typescriptlang.org/docs/handbook/2/mapped-types.html) - [Type predicates](https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates) [Try it out](https://www.typescriptlang.org/play?#code/PTAEEEBEDVwOQMIFFQBUCaAFAknA4gFAigC8Z5FFBRYAMgJYAuApgE4CGANqPQHYusADgHtO7RvWG8aoALTyFipcuqMAnoOag8zRgLjsAtswA84PawPHQzAB4teAEwDOoZ41Z8A5gD5SoAAMvXQASAG8EdkEmLnoAL1NzDytmHwBfAIIAYyl3UF4jZh09NgAubV19QpMAcgLjGr8SUBrgxnrmGuzcxlB2YOKBcsG2FNr+zqaWtvBgrpkAWSjNR1B1TWcZZRUZACJwN11QYQAzUE1WQ1cT1mFDPsPe09AsqPYAI3pOJnpmZwA6XYEdZaSKCD5fH5-fwAJWYOVYjhM7k8vC8ABpQO9hKJmOxeD5gRotJg2FcTAgbPZmE5XGCId8JH8pnCEUiANbMNTPBCY7G4-GEgicI6OZgAN3oWWY9M+jN+rmaYQIoGOml4mAArpxnMxyh5Ncx0SrQABHYTvABW+tYhuNqtN7GchgAzOUTlxdQQ0tQcrw8prdaxSZdFaBlarhOqtTq9aAPbH7WaLdb456jSbHc63WtbcxvdRiPtHsczjSJIwRcZ+NdbvdGAALLQ5QyfAoSKSll5vOVQ1z41YXK6AomaUBIfhMKvlsYAaSpDhcbg83imAVevAAqrrwmCYt8EiZZ+lMiDx5PK8xq4xnBSFzSl7LIUznFMI6AANqzrk8XigTncmclJOueFbTvwYwot4oAAGSgN+ag+AAuuU-IiviBYir0gZsBOYFXuWYZtHhU4ETWAAUYqStKT7yn8mI4cGZLOAAlAQjEkZe14AlGNIxl6HEXuBN7-Bu27MAA8tG2pegQJyarwWQdn+xFCWRN53nYi50j2z4Kj45Emq84K9i+5S8iaQ7OOUIbkgghIseUnHCbe9nhkZPQ2Gp3HlKywiIsiK5onyOLoQS-hhD6qonP5oDkX6eQfsZKSYuKXCGkhXYSVa8KMP85aeH88W6XRrEse5qqqglvQzoU-jrvi4nhMZ+7xMwxWCCkLEZCaqrlqR3EfrVxiZc0VlJVEKSZTBcFpZwhomlFoCsLomqsH+-VcYRfSuM56muYSPpyQpSmSH+LWMLECTkR05RQWijnLqiXgVctq3rfkhSiQ27CsOY5EAAwsf8jDCJuggXJEurkeVADUn3GP8zjfNK5EAIxsUdxCoMS5wrY4UriH8WzbNshZgMWLbgkpmLfF4DaMAA7sw9D070K2CCtur8OIZ1dgAEqgCy0L+8b+fczzsGsmqCCKI5nqg-RwMIYr+B+90YqA-OMIYnDK2KH5IZiKa5Uho5aKgWn61ozQa+bWs63rKs22gSvO6AAA+aBW875OgMW8mKcpazCC8wgKb0jZaLwzvOECgenZ2OQR9ruvW845G3MIjDlKnTtio9vCaq2bCvStjBred4cQc7mc4owmPUAnwfJzXYo3c7ueO9bhfF+8pfvvQZzkfQziW-Y1sdwX5Xvqq5eV6AaOLTYsavVVnlZA2XyOCtf7NDHBto2blVvRXH1o6A8Ot4wCBb5wO80vFd8P7wbGqj6R3N3zo-j4wk8H3GPOPdygAJ4K4X+1sy7vT-CCZ4oDyAtA1l0T+J0W7Vxvs-XeT9t67y7mnZ2hte4l1YFAs+51ME0l6qAf4hgohTxtn4a+-9nYsTfpVf4+NNSo3IuKTEzhi7lRIH4fh9x4a8NAEDb0QA) in the TypeScript playground!

## Limits of expressivity

Some relations [I cannot get them work](https://www.typescriptlang.org/play?#code/PTAEEEBEDVwOQMIFFQBUCaAFAknA4gFAigC8Z5FFBRYAMgJYAuApgE4CGANqPQHYusADgHtO7RvWG8aoALTyFipcuqMAnoOag8zRgLjsAtswA84PawPHQzAB4teAEwDOoZ41Z8A5gD5SoAAMvXQASAG8EdkEmLnoAL1NzDytmHwBfAIIAYyl3UF4jZh09NgAubV19QpMAcgLjGr8SUBrgxnrmGuzcxlB2YOKBcsG2FNr+zqaWtvBgrplUDS1DKOjeL2cZZW3qYgAicDddUGEAM1BNVkNXU9ZhQz6j3rPQLKj2ACN6TiZ6ZmcAHR7AjqTSgSKCT7fX7-fwAJWYOVYjhM7k86wANKAPsJRMx2LwfCClqBMGxriYEDZ7MwnK4IVCfhJ-lMEUiUQBrZhqF4ILE4vEEokETjHRzMABu9CyzAZXyZf1czTCBFAJ00vEwAFdOM5mOUPFrmBjVaAAI7CD4AKwNrCNJrVZvYzkMAGZyqcuHqCGlqDleHktXrWGSrkrQCq1cINdrdfrQJ64w7zZabQmvcbTU6Xe7QIbmD7dmADk8TudaRJGKLjPwbncHowABZaHKGL4FCRSMuvd7ymGuAmOC7kwHA0FaJD8JjVitjADS1IcLjcHm8UwCb14AFU9eEITEfgkTHP0plx6BJ5WZ7XKYvacu5dDmc4ppHQABtOfcni8UBcnnnFSzoXlOVbMDWjBjGi3igAAZKAX5qD4AC65QCqKBKFqKvRBmwl7TuBFbhm0+FgRBzgABTilKMqPgq-xYrhIYjgAlAQTGkdejCAtGtKxt6HGgVxgKbjuzAAPIxjq3oEKcWq8Fkna-iRQmETeVJ2Eu9K9k+io+BRppvJCfbPuUfKmpc1zlKGFIIESLHlJxancZSfhvv6eQVgR5HlGywjIqiq6YtiuIYYS-hhL6aqnP5oAUR5vTvkZKRYhKXBGsh3bidaiKMACFaeP88U6fRzgsSxEammqCU2PwKT+BuBJieERkHvEzDFYIKQsRkVW1VeznOO+s6FJlzSWUNyWjXB8FpZwRqmlFoCsLoWqsL+XlkURfSuE55GuYWsnyYpki-q1jCxAkFEdOU0HrA5K7ol4lVqitjBrb+HQAlkjbsKw5gUQADCxAKMMIW6CJckR6hRFUANT5IUALOD8MoUQAjGxvoyHsqDNj+DhKVwLS8DqnA1KAEpsM4p3duwFzCM4NMfJwag-q2gjYVokJqJwwjsI4Y4knAZOfKKJioFMqCgAAPvkZPEmC2D8F5UhcBLr6mp+358H+3IvKgyGyAA-OUqDa2omWaferh8KcbCgAAyrLoDyeKpx8MwQ7G07oDlLwkpsIdNXCBFprsOU6PJh85QAExYcctjlP+Lzji8wiqCSCC-RwilsJg7C8-zQ7KqaN2Pd4yZiO4psV5iFl3PbTOnbXd1eId2GgFawiNrw5TK4Tp3q9nf3sHnIaF3zAua2q5c1AAUj3vA1FXzqMP7ZPJoIjf-DTUgb5wnCHcQixgtvXvSuI-xbNsOw44cHNj4wWI-F4jaMAA7sw9Bv70K3n3qfg4haYvAABKoAALK0B-AmfyDwXj03epzZgQJFZaFQP0OAwhxT+HfG3LEoDGCGE4Fg8U75kJYlTLlZCaC0CaVIVoZobdaGEOIQw-wGCvDsLlqgeh2CCz3wTMdJSeZQ45Hkr0JsWheD8OcMCOSCkRHiP4KwkhsiKJ3GEOvUAqiGEPVJm2B2b43ofVeMICRDCNG4kYFjagCiTpdmUZBfh11+HlF0fw-RWpDGsBejwc4FF6DOF4fYSxMjxTlT8a9Va61QDo0WjYOMUSzEBl6D9b4jgVq-maOE5g750Y0LVNE96sT0agARk47OGSsnxUbNU2kbE1S+mxvYkRQSQnOPFK48U7iiFqIif7fhPBXAdPYcYmJv407nFyaQMgLQ25dBacI2mlS6mcEybSWp9S+46L6Qw8hXifF+JMbE9J6ysl9QBCsQQ3TGF+CcWEzxjSikAhWo4LUaMKISixM4bxFUSB+F+Q8BG3zQDAx9EAA) in the TypeScript playground!

## Resources

- TypeScript [handbook](https://www.typescriptlang.org/docs/handbook/). - TypeScript [playground](https://www.typescriptlang.org/play).