Typescript assign null to a type don't trigger error

Question

Can anyone explain me, please, why this code dont trigger an error?

import { Injectable } from '@angular/core';

interface Animal{
  name: string;
}

@Injectable()
export class AnimalService {
  lion: Animal = null;
  constructor() {}
  get(){
   return this.lion;
 }
}

By default in JavaScript(and Typescript) any field can have the value null or undefined, have a look at stricktNullCheck. — Adrian Fâciu
– Adrian Fâciu, Commented Aug 13, 2017 at 8:33

Saravana · Accepted Answer · 2017-08-13 08:58:44Z

8

You have to set "strictNullChecks": true in your tsconfig.json for it to throw error when you do lion: Animal = null;.

See the documentation for strictNullChecks compiler flag:

In strict null checking mode, the null and undefined values are not in the domain of every type and are only assignable to themselves and any (the one exception being that undefined is also assignable to void).

Also see: https://ariya.io/2016/10/typescript-2-and-strict-null-checking

edited Aug 13, 2017 at 8:58

answered Aug 13, 2017 at 8:32

Saravana

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5 Comments

Whisher Over a year ago

So what's the best options put the flag to false or set it to true and using a syntax like const lunchBox: Nullable<Food>[] = [];

Saravana Over a year ago

That depends on your requirements. I personally turn on strictNullChecks and use variable: Type | null for specific cases where nulls can be allowed.

Spock Over a year ago

It just becomes a nightmare because now you have to typecast every time you set a value. You cannot set it to a string value because the compiler will say it doesn't match the defined type string | null

mdsimmo Over a year ago

@Spock use the non null assertion operator: !. E.g someNonNullMethod(someNullabeValue!)

Spock Over a year ago

@mdsimmo yep, it just feels like a hack :) .. but yeah good point, thanks

Mohamed A M-Hassan · Accepted Answer · 2021-08-02 06:04:36Z

3

use variable: Type | null for specific cases where nulls can be allowed.

ex: name:string| null

I personally turn on strictNullChecks and use the specific cases.

answered Aug 2, 2021 at 6:04

Mohamed A M-Hassan

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