Incorrect type inferred with early return

[Bibliofile]

When returning early from a function, types are not inferred correctly in functions defined later in the function. This occurs only with strictNullChecks turned on. Playground.

TypeScript Version: typescript@2.6, typescript@2.7.0-dev.20171102

Code

interface hasMethods {
 remove(): void
 getExports(key: string): {[key: string]: any} | undefined
}

let obj: hasMethods = {
 remove: () => { },
 getExports: () => undefined
}

;(function () {
 let x = obj.getExports('test')
 if (!x) return
 // No error, as expected
 x.test()
 // Error, x might be undefined
 obj.remove = () => x.test()
}())

x is a mutable variable. Since x might be mutated at some point, we can't be sure that a reference to x in a callback will still have x defined. (#9998)
In this case you're not actually mutating x, so use const instead. There's a lint rule for that too.