Typescript does not recognize function Boolean() as a check for undefined

I followed the submission guidelines. I prefer to check for a defined object using Boolean(object) rather than !!object, which I consider bad style. Typescript linter recognizes !! as a check for undefined but does not recognize Boolean() as a check for undefined. I could not find a tsconfig or tslint configuration setting related to this behavior. <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> 2.9.0-dev.20180511 (typescript@next) <!-- Search terms you tried before logging this (so others can find this issue more easily) --> "Boolean" "Boolean(" "function Boolean" **Code** This is not production code but a simple demonstration only to satisfy the expression tree. ```ts function booleanTest() { let value: Array<object> | undefined = undefined; if (Date.now() > 1) { value = [{}]; } if (Boolean(value)) { console.log(value.length); } } ``` **Expected behavior:** No Typescript linter errors **Actual behavior:** Typescript linter in VS Code displays an error "[ts] Object is possibly 'undefined'." with a red squiggly on the final reference to variable "value". **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior -->