TypeScript object type assertion doesn’t work

Question

type ConfigItem<T> = {
  label: string;
  name: keyof T;
};

type P1 = {
  E1: {
    A: "xl",
    B: "xl",
    C: "xl",
    D: "xl",
  },
  E2: {
    AA: "xxl",
    BB: "xxl",
    CC: "xxl",
    DD: "xxl",
  },
};

export const Configs = {
  X1: [
    { label: "ax", name: "A2" }, // ⬅️ Why is it not restricted?
    { label: "bx", name: "B" },
    { label: "cx", name: "C" },
    { label: "dx", name: "D2" }, // If the last one is not in E1, an error is reported.
  ] as ConfigItem<P1["E1"]>[],
  X2: [
    // ...
  ]
};

An error is reported only if the last item in the array has the wrong value type.

When the last item is correct, the previous item is incorrect, no error will be reported.

"typescript": "^4.4.4"

[UPDATE]: Is that the only way to do it?

const X1: ConfigItem<P1["E1"]> = [
  { label: "ax", name: "A2" },
  { label: "bx", name: "B" },
  { label: "cx", name: "C" },
  { label: "dx", name: "D" },
];

export const Configs = {
  X1,
  X2: []
};

What behavior are you expect ? Please provide more context to the problem you have — captain-yossarian from Ukraine
– captain-yossarian from Ukraine, Commented Oct 20, 2021 at 10:00
You shouldn't be using a type assertion at all; you want the compiler to check the types, not just trust you: tsplay.dev/we0veW — jonrsharpe
– jonrsharpe, Commented Oct 20, 2021 at 10:01

flx · Accepted Answer · 2021-10-20 10:55:34Z

1

When you are using the as [[typename]] notation no type checking is going to be performed as you are telling the compiler manually that you are certain this is correct.

What you want to do to enable type checking is:

type ConfigItem<T> = {
  label: string;
  name: keyof T;
};

type P1 = {
  E1: {
    A: "xl",
    B: "xl",
    C: "xl",
    D: "xl",
  },
  E2: {
    AA: "xxl",
    BB: "xxl",
    CC: "xxl",
    DD: "xxl",
  },
};

export const Configs: {X1: ConfigItem<P1["E1"]>[], X2: any /*define next here*/} = {
  X1: [
    { label: "ax", name: "A2" },
    { label: "bx", name: "B" },
    { label: "cx", name: "C" },
    { label: "dx", name: "D2" },
  ],
  X2: [
    // ...
  ]
};

edited Oct 20, 2021 at 10:55

answered Oct 20, 2021 at 10:03

flx

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