Typescript does not check type of mapped variable correctly.

[McDic]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms: ts2531 possibly null

Code

interface test2{
    [key: string]: number | null
}

var obj: test2 = {
    aaa: 1234,
    bbb: null,
    ccc: -5.67
}

function check2(obj: test2, key: string){
    if(typeof obj[key] === 'number' && obj[key] !== null){
        console.log(`ok obj[key] + 10 = ${obj[key] + 10}`)
    }
    else{
        console.log('ok otherwise')
    }
}

for(const key in obj){
    check2(obj, key)
}

var a: unknown;

function check3(a: unknown): any{
    if(typeof a === 'number'){
        return a + 10;
    }
}

Expected behavior: No error

Actual behavior: TS2531 happened on function check2's console logging code. (${obj[key] + 10})

Playground Link: Here

Related Issues: #28131

[MartinJohns]

Duplicate of #10530. Type narrowing does not occur for indexed access forms e[k] where k is not a literal.

You can solve it by just storing the value in a variable:

const value = obj[key];
if(typeof value === 'number' && value !== null){
    console.log(`ok obj[key] + 10 = ${value + 10}`)
}

[fatcerberus]

Type narrowing does not occur for indexed access forms e[k] where k is not a literal.

See also #31445 and #34867. At present, the compiler can't safely narrow these expressions because it only cares about the types and doesn't track the specific k variable used. test2[string] casts too wide a net to serve as a type guard.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.