I am JS developer and new to TS. If I have the following line of code:

  let corners = Boolean(checkTypes[userAccess]);

if I change it to this:

  let corners = Boolean(checkTypes![userAccess]);

why is that frowned upon? I had a comment saying non null assertions are not a runtime check so when would they happen?

CodePudding user response：

Non-null assertions are not done at runtime?

Correct, the JavaScript code will be just:

let corners = Boolean(checkTypes[userAccess]);

...and as a result, if checkTypes is nullish, that code will throw.

so when would they happen?

They won't, unless you code them. Type assertions are just that: assertions. You're promising TypeScript that you know that the assertion is true. TypeScript doesn't insert code to check that you're right. You've asserted that you are right.

Some options for you:

1. If you want a checked assertion, you can write an assertion function (sadly not yet documented in the handbook):

   function assertIsNotNullish<T>(
       value: T | null | undefined,
       message?: string,
   ): asserts value is T {
       if (value == null) { // Which checks both `null` and `undefined`
           throw new Error(message ?? `Expected non-nullish value`);
       }
   }
   

   Then your code would be:

   assertIsNotNullish(checkTypes, "checkTypes was unexpectedly nullish");
   let corners = Boolean(checkTypes[userAccess]); // No assertion needed
   

2. Optional chaining and nullish coalescing (both now JavaScript features):

   // Assuming you want `false` if `checkTypes` is nullish
   let corners = Boolean(checkTypes?.[userAccess] ?? false);
   //                              ^^            ^^^^^^^^^^
   

3. You could have a type predicate:

   function isNotNullish<T>(value: T | null | undefined): value is T {
       return value != null; // Which checks both `null` and `undefined`
   }
   

   and then

   let corners = isNotNullish(checkTypes)
       ? Boolean(checkTypes[userAccess])
       : false; // Or `true` if that should be the default