TypeError: Cannot read property 'length' of undefined in watch mode and {allowJS: true}

[lephuongbg]

TypeScript Version: 3.7.2 and 3.8 beta

Expected behavior:

Newer typescript should not crash with existing code.

Actual behavior:

A specific combination of vuex and typescript 3.7.2 with allowJs: true and watch mode causes typescript to crashes with following stack trace:

$ ./node_modules/.bin/tsc --noEmit --watch
/var/app/web/node_modules/typescript/lib/tsc.js:78566
                throw e;
                ^

TypeError: Cannot read property 'length' of undefined
    at Object.find (/var/app/web/node_modules/typescript/lib/tsc.js:207:35)
    at getDeclarationWithTypeAnnotation (/var/app/web/node_modules/typescript/lib/tsc.js:31333:31)
    at isTypeRepresentableAsFunctionNamespaceMerge (/var/app/web/node_modules/typescript/lib/tsc.js:31275:26)
    at serializeMaybeAliasAssignment (/var/app/web/node_modules/typescript/lib/tsc.js:31251:29)
    at serializeVariableOrProperty (/var/app/web/node_modules/typescript/lib/tsc.js:31059:25)
    at serializeSymbolWorker (/var/app/web/node_modules/typescript/lib/tsc.js:30927:25)
    at serializeSymbol (/var/app/web/node_modules/typescript/lib/tsc.js:30894:38)
    at /var/app/web/node_modules/typescript/lib/tsc.js:30875:25
    at Map.forEach (<anonymous>)
    at visitSymbolTable (/var/app/web/node_modules/typescript/lib/tsc.js:30874:33)

A simple js file like this will trigger above bug:

import {createNamespacedHelpers} from 'vuex'

const {mapState} = createNamespacedHelpers(STORE_NS)

export default {
    computed: {
        ...mapState(['panels'])  // <==== Bug is triggered with this line
    }
}

Reproduction repository is here: https://github.com/lephuongbg/typescript-bug-repro

Related Issues:
Not sure