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TypeScript can't parse optional object when assigned to a variable #36510

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bnbarak opened this issue Jan 29, 2020 · 2 comments · Fixed by #44730
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TypeScript can't parse optional object when assigned to a variable #36510

bnbarak opened this issue Jan 29, 2020 · 2 comments · Fixed by #44730
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@bnbarak
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bnbarak commented Jan 29, 2020

TypeScript Version: 3.7.5

Code

type A = { b?:  { c?: number } }
const a: A = { b: {c: 123} };
const isSafe = a && a.b && a.b.c
// if (a && a.b && a.b.c) { // <= works
if (isSafe) { // <= breaks
    console.log(a.b.c); // (TS 2532) Object is possibly 'undefined'.
}

Expected behavior:
TS should check if isSafe protects against undefined
Actual behavior:
TS doesn't check isSafe

Output 
"use strict";
const a = { b: { c: 123 } };
const isSafe = a && a.b && a.b.c;
// if (a && a.b && a.b.c) { // <= works
if (isSafe) { // <= breaks
    console.log(a.b.c); // (TS 2532) Object is possibly 'undefined'.
}
Compiler Options 
{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided
@nmain100
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Duplicate of #12184

@RyanCavanaugh RyanCavanaugh added the Duplicate An existing issue was already created label Feb 3, 2020
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This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

@ahejlsberg ahejlsberg mentioned this issue Jun 24, 2021
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Control flow analysis of aliased conditional expressions and discriminants microsoft/TypeScript
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@bnbarak @RyanCavanaugh @typescript-bot @nmain100