Microsoft/TypeScript

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

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#24,063 opened on May 11, 2018

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Description

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.

2.9.0-dev.20180511 (typescript@next)

"Boolean" "Boolean(" "function Boolean"

Code This is not production code but a simple demonstration only to satisfy the expression tree.

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".

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Contributor guide

Tech stack
typescript
Domain
developer experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript type systemcontrol flow analysis
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
The issue reports that TypeScript's control flow analysis does not recognize `Boolean(value)` as a type guard, unlike `!!value`. To fix, investigate the handling of `Boolean` function calls in the type checker (likely in `src/compiler/checker.ts`). Look for how `!!` is handled and extend similar logic to `Boolean()`. Check if there's a `isBooleanCall` or similar helper. Consider supporting `Boolean` as a narrowing function. Review related issues (e.g., #24063) and any linked PRs for prior attempts.