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.