Should Limit number based off if guard

[AndrewOttavianoAbb]

🔎 Search Terms

if guard for number

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about if guards.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAsiDKkDGBLAhgGwHIFcC2UAvFAAxQA+UAjBVAEy0DMA3AFCtID2AdgM7BQ8IAIIAnUUSgBtAEQpgEPFRkAaKHIV46q9fMWMZAXTYYIAoQDVMALlgJk6bPkkk2KAGZQAFELGiAdKbcAObAABZQADzEjACUUADerFCCIFYYkr7i-u4o3AAmAJIFEAAeXrxEAHxQlYT1upra8QDU1MxQAPSdUAAq4NAQ4pyirAC+rEA

💻 Code

type MySpecialNum = 0 | 1 | 2 | 3;

const myArr = ["item1", "item2", "item3"];
let myVal: MySpecialNum = 0;
if (myArr.length <= 3) {
  myVal = myArr.findIndex(s => s === "item2") + 1; // Type error
}

🙁 Actual behavior

Type error on this line
myVal = myArr.findIndex(s => s === "item2") + 1; // Type error

🙂 Expected behavior

No type error.

I would expect no type error here because TypeScript has some really good type inference around if branches and typeof. This is really handy I would expect it to handle the problem above. Here is why I think TS should handle the case above:

• I have declared my type to limit my number to the range 0-3 (which is another feature I'd love to see added)
• Then I am creating a variable to find what position in my list a certain element is and I'm using my custom type
• I have an if check to determine if I have less than 3 items in my array so that I can use findIndex to get the index of my element and add one to get the position in the list (note this still breaks if I remove the + 1 at the end

Ordinarily, one might say that findIndex could return something above my limiting type; that is true except I have an if statement guaranteeing that won't be a problem. In a sense, it is a kind of guard.

The main counter argument to this might be that we can't do anything about the arbitrary + 1 at the end. This is where my very limited knowledge of how TS works shows. I'm not sure if it would be possible to account for that, but I wouldn't think it impossible.

Additional information about the issue

No response

[RyanCavanaugh]

This is at least two (probably more) feature requests in one: #26382, #54925 at a minimum.

[typescript-bot]

This issue has been marked as "Too Complex" and has seen no recent activity. It has been automatically closed for house-keeping purposes.