TypeScripting the technical interview

Post by eyelidlessness » Mon, Mar 13, 2023, 4:47 PM UTC

consilient wrote: post #130

eyelidlessness wrote: post #126

Earlier quoted context omitted.

This is a good example of how `never` is treated as `never`. Everything is assignable to the bottom type and intersecting with it will always be the bottom type, by definition. It’s also a good example of how the top type `any` casts to whatever you choose, because that’s also by design. Both types are vacant, the bottom type is infectious. That’s a good thing. And it works the same way with `unknown`, which it also…

> Everything is assignable to the bottom type Other way around: everything is assignable to the top type (`unknown`), and the bottom type (`never`) is assignable to everything. > It’s also a good example of how the top type `any` casts to whatever you choose Which is precisely why `any` isn't the top type: if you allow `any`, then types no longer form a lattice, and there is no bottom or top. If you restrict yourself…

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Post by beebeepka » Mon, Mar 13, 2023, 5:51 PM UTC

TheCleric wrote: post #88

oblak wrote: post #25

Earlier quoted context omitted.

Use "unknown" and TS complains that you're treating something as an object. Use Object.defineProperties and TS complains because that stuff is invisible to it after how many years? I think you're right, of course, but TS is hardly perfect and treating its ways as gospel is not an improvement over JS. The "right ways" change over time and beliefs are not shared among everyone.

If you know what's in the object cast it as that type or be sure by saying if ('propertyName' in unknownObject). TS is far from perfect. These aren't its ways (it provides any, so of course it's fine with it). These are my restrictions: if you're using a type system, actually use it. Don't lie to yourself and throw anys in your code.

Post by ycitm » Mon, Mar 13, 2023, 6:07 PM UTC

Post by TheCleric » Mon, Mar 13, 2023, 9:08 PM UTC

beebeepka wrote: post #164

TheCleric wrote: post #88

Earlier quoted context omitted.

If you know what's in the object cast it as that type or be sure by saying if ('propertyName' in unknownObject). TS is far from perfect. These aren't its ways (it provides any, so of course it's fine with it). These are my restrictions: if you're using a type system, actually use it. Don't lie to yourself and throw anys in your code.

any is the last resort. How do you get around properties assigned via Object.defineProperties recognised by TS, though? I really don't know. Is is an unrelated question

    interface IOptionsX {
        x: number;
    }
    
    interface IOptionsY {
        y: number;
    }
    
    const testObj: IOptionsX = {x : 0};
    
    Object.defineProperties(testObj, {
        y: {
            value: 100,
            writable: true
        },
    });
    
    if ('y' in testObj) { // Could also do `&& typeof testObj.y === 'number'` to be VERY sure.
        const testObjWithY = testObj as typeof testObj & IOptionsY;
        console.log(testObjWithY.y)
    }

Post by TheCleric » Tue, Mar 14, 2023, 2:46 AM UTC

TheCleric wrote: post #168

beebeepka wrote: post #164

Earlier quoted context omitted.

any is the last resort. How do you get around properties assigned via Object.defineProperties recognised by TS, though? I really don't know. Is is an unrelated question

Here's how I would do it: interface IOptionsX { x: number; } interface IOptionsY { y: number; } const testObj: IOptionsX = {x : 0}; Object.defineProperties(testObj, { y: { value: 100, writable: true }, }); if ('y' in testObj) { // Could also do `&& typeof testObj.y === 'number'` to be VERY sure. const testObjWithY = testObj as typeof testObj & IOptionsY; console.log(testObjWithY.y) }