Typescript assert function to avoid Non-Null Assertion Operator ! with Objects

Question

based on this post Typescript - What is the best way to type check an object properties and throw upon null?

I'm trying to check if values of my object are not undefined and narrow the type of the argument to a version with all non-null properties but I keep having this issue: "Argument of type '{ Id?: string | undefined; Name?: string | undefined; }' is not assignable to parameter of type 'MyTypeNotNull'."

I would like to know what's wrong with my assert function signature ?

type MyType = {
    Id?: string | undefined
    Name?: string | undefined
};

type MyTypeNotNull = {
    Id: string
    Name: string
};

function logMyType(type: MyTypeNotNull) {
    console.log(type);
}

function assertNoPropsAreUndefined<T extends object>(obj: T): asserts obj is
    { [K in keyof T]: NonNullable<T[K]> } {
    Object.entries(obj).forEach(([k, v]) => {
        console.log(k, v)
        if (v === undefined)
            throw new Error("OH NOEZ, PROP \"" + k + "\" IS NULL");
    });
}

const run = (value: MyType) => {
    value; // (parameter) value: MyType
    assertNoPropsAreUndefined(value)
    value;
    logMyType(value); // should be ok but it throw Argument of type '{ Id?: string | undefined; Name?: string | undefined; }' is not assignable to parameter of type 'MyTypeNotNull'.
};

run({ Id: 'myKey1', Name: "ok" }); // {key1: "myKey1"}

run({ Id: 'myKey1', Name: undefined }); // 💥 OH NOEZ, PROP "key3" IS NULL

Playground link to code

NonNullable means it's not nullable. It doesn't mean that an optional property becomes mandatory. — VLAZ
– VLAZ, Commented Oct 13, 2022 at 15:50

nathanagez · Accepted Answer · 2022-10-13 15:58:45Z

1

Thanks @caTS. For others in the same situation I was missing the Required built-in type.

type MyType = {
    Id?: string | undefined
    Name?: string | undefined
};

type MyTypeNotNull = {
    Id: string
    Name: string
};

function logMyType(type: MyTypeNotNull) {
    console.log(type);
}

function assertNoPropsAreUndefined<T extends object>(obj: T): asserts obj is
    Required<{ [K in keyof T]: NonNullable<T[K]> }> {
    Object.entries(obj).forEach(([k, v]) => {
        if (v === undefined)
            throw new Error("OH NOEZ, PROP \"" + k + "\" IS NULL");
    });
}

const run = (value: MyType) => {
    value; // (parameter) value: MyType
    assertNoPropsAreUndefined(value)
    value;
    logMyType(value);
};

run({ Id: 'myKey1', Name: "ok" }); // {key1: "myKey1"}

run({ Id: 'myKey1', Name: undefined }); // 💥 OH NOEZ, PROP "Name" IS NULL

Link to playground

answered Oct 13, 2022 at 15:58

nathanagez

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4 Comments

VLAZ Over a year ago

Required<{ [K in keyof T]: NonNullable<T[K]> }> is excessive, you just need Required<T>

k.tten Over a year ago

@VLAZ Then null would pass through. I'm not sure if they intended for null to also be excluded but that is what NonNullable does.

VLAZ Over a year ago

@caTS ah, I missed that null wasn't needed. I was just going off MyType. In that case, it would be shorter to use { [K in keyof T]-?: NonNullable<T[K]> } (the extra -? to make properties not optional) than Required. Not much difference in bytes but it's an option.

nathanagez Over a year ago

I prefer to use Required instead of -? in term of readability. Thanks guys, I learned new things today

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