Type Narrowing Inside Looping Structure Conditions not Persisting

[xXCrash2BomberXx]

Bug Report

🔎 Search Terms

Looping, Condition, Inference, Chain, And, &&, Array

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Type Guards

⏯ Playground Link

Playground link with relevant code

💻 Code

let arr: Array<Array<boolean> | number> = [[true]];

for (let i = 0; i < arr.length; i++)
    if (arr[i] instanceof Array && arr[i].length > 0)  // Property 'length' does not exist on type 'number | boolean[]'.
        console.log(arr[i].length);  // Property 'length' does not exist on type 'number | boolean[]'.

if (arr[0] instanceof Array && arr[0].length > 0)
    console.log(true);

🙁 Actual behavior

When attempting to compile, the condition arr[i] instanceof Array infers that arr[i] is an Array and has the property length but this inference is not continued throughout the rest of the condition nor body within the condition due to the variable i. This behavior does not match the behavior with a fixed-index as is shown in the fixed-index block.

🙂 Expected behavior

Ideally, the inference from arr[i] instanceof Array would be continued into arr[i].length > 0 due to it being an && which makes the second condition never run unless arr[i] is an instance of Array. This should also be the case for the body of the if statement. This behavior is shown with a fixed index but does not apply to a variable one.

[fatcerberus]

It has nothing to do with the loop. It's just that obj[key] isn't narrowed when key is not a literal. Duplicate of #51368 -> #10530

[fatcerberus]

Workaround: First assign the value of arr[i] to a variable inside the loop and narrow that.

[typescript-bot]

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