How do I convince TypeScript that yes, a value is initialised, actually.

[daring roost]

I have some javascript code that essentially boils down to something like

let apple: number;
let banana;
eval("apple = 1;");
banana = apple;

Which gives an error like Variable 'apple' is used before being assigned.ts(2454)

Is there a magic incantation I can use to convince typescript that apple is actually assigned? Maybe a chain of as's or something?

[dusk wyvern]

let apple: number;
let banana;
eval("apple = 1;");
banana = apple!; // notice the !

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-0.html#non-null-assertion-operator

[daring roost]

Great, thanks! I can't believe that didn't show up in my google searches

[fading wigeon]

let apple!: number;
let banana;
eval("apple = 1;");
banana = apple;

[clever totem]

this is the more common solution

also works for class fields

the issue with the operator form is that it only tells TS it's initialized in that one specific place

[daring roost]

Cool. Is that syntax in the docs somewhere?

[clever totem]

i guess it is https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html#definite-assignment-assertions

super out of the way though

i had to do https://www.google.com/search?q="!%3A"+site%3Atypescriptlang.org%2Fdocs

[daring roost]

Nice, good find! I was having trouble with it too. Maybe it needs a pull request to add it to the docs proper?

Or an issue