CODEWITHSUNDEEP
typescript
mikemaccana
mikemaccana

Reputation: 123560

Why is TypeScript not warning when setting a non-nullable value as null?

I am specifying types on a function. TypeScript types are (as of current TypeScript versions) non-nullable by default. However I don't get any errors when I run a function with null or undefined.

function sayHello(name: string){
  console.log(`Hello ${name}`)
}

In the code above sayHello(undefined) and sayHello(null) should both fail.

What they currently do in TypeScript 3.8.3: 

Hello null
Hello undefined

vsCode gives no warnings:

enter image description here

Why is TypeScript not warning when setting a non-nullable value as null?

Upvotes: 7

Views: 2599

Answers (2)

Klas Mellbourn
Klas Mellbourn

Reputation: 44417

Not nullable is now the default in TypeScript. You should get the typescript compilation error:

Argument of type 'null' is not assignable to parameter of type 'string'. ts(2345)

If you don't get that, you may want to take a look at your compiler options. I would recommend the strict option.

You can set that up with a tsconfig.json file containing something like this: 

{
  "compilerOptions": {
    "strict": true,
  },
}

Upvotes: 3

Kenan Güler
Kenan Güler

Reputation: 2013

Do you have a tsconfig.json file? If so, how does it look like?

Make sure, you have following entries in your tsconfig.json (shortened for the sake of brevity):

{
  "compilerOptions": {
   "strictNullChecks": true
  }
}

From the TypeScript doc:

strictNullChecks: switches to a new strict null checking mode.

Upvotes: 8

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