tsc doesn't catch use of unassigned variable inside closure

[steabert]

TypeScript Version: 3.6.0-dev.20190621

Search Terms:

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
let a: number[]

const b = () => {
  a.push(2) // no 2454 error
}

b() // runtime error

Expected behavior:

Compile time error 2454.

Actual behavior:

No error during compiling with tsc, runtime error instead.

Playground Link:

Related Issues:

Not really, i found a bunch of bugs related to use before assign, but not this. It seems this should be a fairly obvious bug, or I'm doing something wrong.

[nmain]

Since the error is caught when b is replaced by an IIFE, isn't this just #9998?

[steabert]

I'm not sure about #9998, I certainly didn't immediately make the connection. However, trying to work around it by allowing a to be undefined:

let a: number[] | undefined = undefined

a.push(2)

made me even more confused as this doesn't get caught even when not in a closure. It seems I'm doing something wrong here.

[nmain]

For that one, you need to turn on strictNullChecks.

[steabert]

Thanks! I had that in my tsconfig.json but missed that my test file wasn't included there. I'll use it as a work-around, then I guess this can be closed as duplicate of / related to #9998?

[ajafff]

Variables that are never assigned are likely a bug. I can offer a lint rule to detect such variables: https://github.com/fimbullinter/wotan/blob/master/packages/mimir/docs/no-unassigned-variable.md

[typescript-bot]

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