TypeScript !. meaning

Question

I was skimming through some documentation in the Angular docs and ran across this chunk of code:

return this.exampleDatabase!.getRepoIssues(
                this.sort.active, this.sort.direction, this.paginator.pageIndex);

Is this the TypeSrcript equivalent of the C# null-conditional operator for null checking?

I tried doing some stuff in JSFiddle but kept getting syntax errors:

Example:

const obj = { test: 'ing 123' };

console.log(obj.test);
console.log(obj!.doesntexist); // Excepting null here

But, instead, I get:

Uncaught SyntaxError: Unexpected token . at new Function () at exec (typescript.js:41) at HTMLDocument.runScripts (typescript.js:41)

Duplicate of [In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?](https://stackoverflow.com/questions/42273853/in-typescript-what-is-the-exclamation-mark-bang-operator-when-dereferenci) — DrZoo, Sep 05 '19 at 19:22

lukasgeiter · Accepted Answer · 2019-09-05T19:33:00.600

4

That's the not-null-assertion operator. It basically tells typescript that the expression before it is not null and not undefined.

It's supposed to be used in cases where you as the developer are sure that a variable is defined but the compiler is unable to conclude that fact.

Looking into why this results in a syntax error on JSFiddle...

The transpiled code it's trying to run (using new Function(transpiled)()) contains this:

console.log(obj, !.doesntexist);

I couldn't find out what version of TypeScript they're using exactly, but it must be older than 2.0 when the operator was introduced.

edited Sep 05 '19 at 19:33

answered Sep 05 '19 at 19:19

lukasgeiter

Thanks for the link. One question, why do I end up with a syntax error in my basic example? – mwilson Sep 05 '19 at 19:21
Because you're using TypeScript syntax in JavaScript? – jcalz Sep 05 '19 at 19:22
No, I've got JSFiddle set to use TypeScript – mwilson Sep 05 '19 at 19:22
@mwilson Could you link the JSFiddle in your question? – lukasgeiter Sep 05 '19 at 19:23
https://jsfiddle.net/mswilson4040/nowthced/1/ – mwilson Sep 05 '19 at 19:25
Maybe JSFiddle is using some archaic version of TS. I haven't tried this code anywhere else. – mwilson Sep 05 '19 at 19:27
1

@mwilson Yeah looks like they're using an old version (lower than 2.0). See the updated answer with some details. – lukasgeiter Sep 05 '19 at 19:33
If you don't need HTML or CSS I suggest the [official TypeScript playground](http://www.typescriptlang.org/play) as an alternative to JSFiddle – lukasgeiter Sep 05 '19 at 19:36

1 Answers1