Merge pull request #9422 from Microsoft/allow-destructuring-assignment-with-default-of-undefined-optional-property

Destructuring assignment removes undefined from type when default value is given

Author: mhegazy

src/compiler/checker.ts

@@ -12544,6 +12544,12 @@ namespace ts {
             if (exprOrAssignment.kind === SyntaxKind.ShorthandPropertyAssignment) {
                 const prop = <ShorthandPropertyAssignment>exprOrAssignment;
                 if (prop.objectAssignmentInitializer) {
+                    // In strict null checking mode, if a default value of a non-undefined type is specified, remove
+                    // undefined from the final type.
+                    if (strictNullChecks &&
+                        !(getCombinedTypeFlags(checkExpression(prop.objectAssignmentInitializer)) & TypeFlags.Undefined)) {
+                        sourceType = getTypeWithFacts(sourceType, TypeFacts.NEUndefined);
+                    }
                     checkBinaryLikeExpression(prop.name, prop.equalsToken, prop.objectAssignmentInitializer, contextualMapper);
                 }
                 target = (<ShorthandPropertyAssignment>exprOrAssignment).name;

tests/baselines/reference/destructuringAssignmentWithDefault.js

@@ -0,0 +1,11 @@
+//// [destructuringAssignmentWithDefault.ts]
+const a: { x?: number } = { };
+let x = 0;
+({x = 1} = a);
+
+
+//// [destructuringAssignmentWithDefault.js]
+var a = {};
+var x = 0;
+(_a = a.x, x = _a === void 0 ? 1 : _a, a);
+var _a;

tests/baselines/reference/destructuringAssignmentWithDefault.symbols

@@ -0,0 +1,12 @@
+=== tests/cases/compiler/destructuringAssignmentWithDefault.ts ===
+const a: { x?: number } = { };
+>a : Symbol(a, Decl(destructuringAssignmentWithDefault.ts, 0, 5))
+>x : Symbol(x, Decl(destructuringAssignmentWithDefault.ts, 0, 10))
+
+let x = 0;
+>x : Symbol(x, Decl(destructuringAssignmentWithDefault.ts, 1, 3))
+
+({x = 1} = a);
+>x : Symbol(x, Decl(destructuringAssignmentWithDefault.ts, 2, 2))
+>a : Symbol(a, Decl(destructuringAssignmentWithDefault.ts, 0, 5))
+

tests/baselines/reference/destructuringAssignmentWithDefault.types

@@ -0,0 +1,17 @@
+=== tests/cases/compiler/destructuringAssignmentWithDefault.ts ===
+const a: { x?: number } = { };
+>a : { x?: number | undefined; }
+>x : number | undefined
+>{ } : {}
+
+let x = 0;
+>x : number
+>0 : number
+
+({x = 1} = a);
+>({x = 1} = a) : { x?: number | undefined; }
+>{x = 1} = a : { x?: number | undefined; }
+>{x = 1} : { x?: number; }
+>x : number
+>a : { x?: number | undefined; }
+

tests/cases/compiler/destructuringAssignmentWithDefault.ts

@@ -0,0 +1,4 @@
+// @strictNullChecks: true
+const a: { x?: number } = { };
+let x = 0;
+({x = 1} = a);