TypeScript warning/errors

[towry]

Type 'string' cannot be used to index type 'T'

microsoft/TypeScript#31661

[towry]

协变, 逆变

ITreeNode 是一个 interface，FormatterRenderItem 是一个实现，但是增加了其他的属性。这个callback函数会被调用者传入一个实现 ITreeNode 接口的类型，但是我们定义这个callback的时候，给的是 FormatterRenderItem。但是 ITreeNode 类型的对象是不能赋值给 FormatterRenderItem 类型的对象，因为前者缺少一些后者定义的属性。我们定义了一个和FormatterRenderItem工作的callback，但是给callback传递参数的行为我们无法控制，假如传递了一个另外一个实现ITreeNode的类型对象，可能会在 callback 里报错的，因为我们可能在callback里调用了 <FormatterRenderItem>.test()，这个 test ITreeNode 没有定义，其他实现者可能也不会定义，就算定义了，行为也可能不一样。

---

• https://stackoverflow.com/questions/51767338/what-is-the-benefit-of-using-strictfunctiontypes-in-typescript
• https://www.typescriptlang.org/docs/handbook/type-compatibility.html#function-parameter-bivariance
• 👍 https://medium.com/@michalskoczylas/covariance-contravariance-and-a-little-bit-of-typescript-2e61f41f6f68
• 😀 https://codewithstyle.info/Strict-function-types-in-TypeScript-covariance-contravariance-and-bivariance/
• Allow type system to represent this in function inputs/outputs microsoft/TypeScript#285 (comment)

[towry]

microsoft/TypeScript#283 (comment)

---

(JSDoc) Workaround / for future reference: use manual cast /** @type {name} */ (ref).

// cloneNode produces Node, @see https://github.com/Microsoft/TypeScript/issues/283
var a = document.cloneNode(true);
a.location; // Property 'location' does not exist on type 'Node'.
// workaround:
var b = /** @type {Document} */ (document.cloneNode(true));
b.location; // OK

[towry]

'this' type is available only in a non-static member of a class or interface.

The wrong code:

class Base {
    hooks: {
        // raise error here.
        self: SyncHook<[this]>;
    }
}

The correct code:

interface Hooks<C> {
    self: SyncHook<[C]>;
}

class Base {
    hooks: {
    } & Hooks<this>;
}

[towry]

Get type of generic property.

type Foo<T> = T extends { NodeC: infer U } ? { NodeC: U } : never;

class LinkedList {
    private dummyNode: Foo<this>["NodeC"];

    constructor() {
        const NodeC = this.NodeC;
        this.dummyNode = new NodeC();
    }

    private NodeC = class {
        constructor() {

        }
    }
}

const list = new LinkedList();
console.log(list);