CODEWITHSUNDEEP
typescript
Qwertiy
Qwertiy

Reputation: 21380

Undefined after a check?

in following code with strictNullChecks turned on I'm getting an error

Object is possibly 'undefined'.

class Smth {
    private data: Array<{ value: number} | undefined> = [];

    public doSmth(i: number) { 
        const data = this.data;
        return data[i] && data[i].value === 0
        //                ^^^^^^^ Object is possibly 'undefined'.
    }
}

I can't understand how it can be undefined after a check.

Upvotes: 4

Views: 313

Answers (1)

CRice
CRice

Reputation: 32146

This is a known issue in Typescript.

TL;DR of that issue, Typescript can make these checks but the devs have chosen not to due to a significant performance cost on the compiler.

The workaround is to use the non-null assertion operator (!) to tell the compiler that the value is not null or undefined. For this example:

return data[i] && data[i]!.value === 0

Upvotes: 1

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