When verifying for null in an instance method, an error of "Potential null object" is encountered

Question

When verifying for null in an instance method, an error of "Potential null object" is encountered

If I create an instance function that checks for a non-null property in TypeScript, I encounter an error stating 'Object possibly null' when using the function in a conditional statement. However, if I directly check for null, the error does not appear. How can I resolve this issue?

type SomeType = {
     someFunc: Function;
}

class A {
    bar: SomeType | null;
    constructor() {
      this.bar = null;
    }
    hasBar() {
      return this.bar !== null;
    }

}

const a = new A();

if (a.hasBar()) {
    a.bar.someFunc(); // Throws object possibly null error
}

if (a.bar !== null) {
    a.bar.someFunc(); // Does not throw any errors
}

Access TypeScript playground Here (Enable strictNullChecks in options)

typescript

Answer 1

Answer №1

One limitation of flow analysis is that checks inside functions do not impact the code outside of the function. For more information on this, refer to this issue.

The only type of function that can actually affect flow analysis is a type guard:

function hasValue<T>(v: T| null): v is T {
    return v !== null;
}
const a = new A();

if (hasValue(a.bar)) {
    a.bar.someFunc(); // a.bar is not null
}

Answer 2

One limitation of flow analysis is that checks inside functions do not impact the code outside of the function. For more information on this, refer to this issue.

The only type of function that can actually affect flow analysis is a type guard:

function hasValue<T>(v: T| null): v is T {
    return v !== null;
}
const a = new A();

if (hasValue(a.bar)) {
    a.bar.someFunc(); // a.bar is not null
}

When verifying for null in an instance method, an error of "Potential null object" is encountered

Answer №1

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