JavaScript for People Who Hate JavaScript

Re:JavaScript for People Who Hate JavaScript by smaddox on Wednesday August 09, 2017 @12:44PM (#14971035)

I encountered this the other day, while playing with typescript for the first time. Is there a good solution, such as a utility function that does a runtime check before casting? I suppose I could write one for each type cast, but it seems like I shouldn't need to.

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Re:JavaScript for People Who Hate JavaScript by styfle on Wednesday August 09, 2017 @01:40PM (#14971669)

Yes, a runtime check in TypeScript is the same as a runtime check in JavaScript.

    function inc(n) {
      if (typeof n === 'number') { return n + 1 }
      throw new Error('Parameter n is not a number');
    }

If you wanted to actually runtime check all types (probably not advised) you could parse the type annotations.

Or you could define your types using something like JSON Schema and validate against that.

https://github.com/bcherny/json-schema-to-typescript

https://github.com/YousefED/typescript-json-schema

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Re:JavaScript for People Who Hate JavaScript by slackingoff2017 on Wednesday August 09, 2017 @06:54PM (#14974523)

I'm sorry to say but... This check won't help with accidental incorrect casts. Typeof in typescript is a pass through for the JavaScript function, so it only returns what the JS version does.

A check for a string or number will work, but a check against a typescript object or something complex like a union type will just return 'Object'

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Re:JavaScript for People Who Hate JavaScript by spion on Saturday August 19, 2017 @11:23AM (#15053885)

You can use a tslint rule to block casts. That way you will only be able to use type guards to change the type of something.

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Re:JavaScript for People Who Hate JavaScript by slackingoff2017 on Wednesday August 09, 2017 @01:19PM (#14971414)

Kind of maybe. See http://blog.wolksoftware.com/decorators-reflection-javascrip... for use of experimental reflection API.

Note that Typescript tries to follow JS to remain a superset that easily compiles down, and the metadata API is currently a stage II Ecma proposal so it's marked experimental.

It should be fairly easy to build a types casting method using that.

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