How do JavaScript unit-tests differ from their TypeScript equivalent?

Question

I'm used to dynamic languages, but TypeScripts structural typing sounds like an almost dynamic addition to JavaScript.

Now I read, TypeScript would eliminate many unit-tests a JavaScript program needs.

What kind of unit-tests are essential for TypeScript 2.0?

What errors can't the type checker get?

(I'm asking for TS2, because I guess the new additions of flow-analysis and non-nullable-types eliminate their own kind of tests)

basarat · Accepted Answer · 2016-10-02 05:54:47Z

1

What errors can't the type checker get

The ones that aren't expressed in the type system. e.g. the length of a character isn't expressed:

let x: string; 
x = theValueThatWasReceivedFromServer;
// read the 10th character 
// As far as ts is concerned it is okay. 
// It might be undefined based on the value that came from the server
console.log(x[10]);

answered Oct 2, 2016 at 5:54

basarat

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Ray · Accepted Answer · 2016-10-02 06:12:30Z

1

What kind of unit-tests are essential for TypeScript 2.0?

Ask yourself,

What kind of unit-tests are essential for Javascript?

There are an infinite number of errors that Typescript can't catch. TypeScript will eliminate a lot of errors that dynamic languages can't, so that leaves (infinity - some = infinity) errors left for you to check. Good Luck! :}

answered Oct 2, 2016 at 6:12

Ray

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