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Merged
ahejlsberg merged 2 commits into from
May 2, 2026
Merged

Port JS specific code for this.xxx assignment declaration typing #3680

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f156f95
Port JS specific code for this.xxx assignment declaration typing
ahejlsberg May 1, 2026
b919d61
Add regression tests
ahejlsberg May 1, 2026
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46 changes: 38 additions & 8 deletions internal/checker/checker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -17699,10 +17699,11 @@ func (c *Checker) getWidenedTypeForAssignmentDeclaration(symbol *ast.Symbol) *Ty
t = c.getTypeFromTypeNode(declaration.Type())
break
}
assignedType := c.getAssignmentDeclarationInitializerType(declaration)
// We ignore initial assignments of undefined to CommonJS exports when there are multiple assignment declarations
if ast.GetAssignmentDeclarationKind(declaration) != ast.JSDeclarationKindExportsProperty || i != 0 || len(symbol.Declarations) == 1 || assignedType.flags&TypeFlagsUndefined == 0 {
types = core.AppendIfUnique(types, assignedType)
if assignedType := c.getAssignmentDeclarationInitializerType(declaration); assignedType != nil {
// We ignore initial assignments of undefined to CommonJS exports when there are multiple assignment declarations
if ast.GetAssignmentDeclarationKind(declaration) != ast.JSDeclarationKindExportsProperty || i != 0 || len(symbol.Declarations) == 1 || assignedType.flags&TypeFlagsUndefined == 0 {
types = core.AppendIfUnique(types, assignedType)
}
}
}
if kind == thisAssignmentDeclarationMethod && len(types) > 0 {
Expand All @@ -17711,7 +17712,10 @@ func (c *Checker) getWidenedTypeForAssignmentDeclaration(symbol *ast.Symbol) *Ty
}
}
if t == nil {
t = c.getUnionType(types)
t = c.anyType
if len(types) != 0 {
t = c.getUnionType(types)
}
}
}
t = c.getWidenedType(t)
Expand All @@ -17726,16 +17730,42 @@ func (c *Checker) getWidenedTypeForAssignmentDeclaration(symbol *ast.Symbol) *Ty

func (c *Checker) getAssignmentDeclarationInitializerType(node *ast.Node) *Type {
if ast.IsBinaryExpression(node) {
var t *Type
switch ast.GetAssignmentDeclarationKind(node) {
case ast.JSDeclarationKindModuleExports, ast.JSDeclarationKindExportsProperty:
return c.getRegularTypeOfLiteralType(c.checkExpressionCached(ast.GetRightMostAssignedExpression(node)))
t = c.getRegularTypeOfLiteralType(c.checkExpressionCached(ast.GetRightMostAssignedExpression(node)))
case ast.JSDeclarationKindThisProperty:
if c.containsSameNamedThisProperty(node.AsBinaryExpression().Left, node.AsBinaryExpression().Right) {
return nil
}
fallthrough
default:
t = c.checkExpressionForMutableLocation(node.AsBinaryExpression().Right, CheckModeNormal)
}
if c.isEmptyArrayLiteralType(t) {
c.reportImplicitAny(node, c.anyArrayType, WideningKindNormal)
return c.anyArrayType
}
return c.checkExpressionForMutableLocation(node.AsBinaryExpression().Right, CheckModeNormal)
return t
}
if ast.IsCallExpression(node) {
return c.getTypeFromPropertyDescriptor(node.Arguments()[2])
}
return c.neverType
return nil
}

func (c *Checker) containsSameNamedThisProperty(thisProperty *ast.Node, expression *ast.Node) bool {
var visit func(node *ast.Node) bool
visit = func(node *ast.Node) bool {
if c.isMatchingReference(thisProperty, node) {
return true
}
if ast.IsFunctionLike(node) {
return false
}
return node.ForEachChild(visit)
Comment on lines +17763 to +17766
}
return visit(expression)
}

func (c *Checker) getTypeFromPropertyDescriptor(node *ast.Node) *Type {
Expand Down
81 changes: 81 additions & 0 deletions testdata/baselines/reference/compiler/thisPropertyAssignmentTyping.errors.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,81 @@
main.js(47,21): error TS2532: Object is possibly 'undefined'.
main.js(55,9): error TS2322: Type 'string' is not assignable to type 'number'.
main.js(64,9): error TS7008: Member 'bar' implicitly has an 'any[]' type.


==== main.js (3 errors) ====
class C1 {
constructor() {
this.foo = [3];
this.foo = [this.foo[0] * 2];
this.foo; // number[]
}
}

class C2 {
constructor() {
this.foo = [3];
this.foo; // number[]
}
method1() {
this.foo = [this.foo[0] * 2];
}
}

class C4 {
constructor() {
this.bar = [];
this.bar.push("baz");
this.bar; // string[]
}
}

// When this.xxx assignment declarations occur only in methods, the declared type of the
// property is a union of the types of all assigned values, excluding values from self-referential
// assignments, plus undefined. Local CFA removes the undefined when possible.

class C5 {
method1() {
this.foo; // number[] | undefined
this.foo = [3]
this.foo = [this.foo[0] * 2]
this.foo; // number[]
}
}

class C6 {
method1() {
this.foo = [3];
this.foo; // number[]
}
method2() {
this.foo; // number[] | undefined
this.foo = [this.foo[0] * 2]; // Error: Object is possibly undefined
~~~~~~~~
!!! error TS2532: Object is possibly 'undefined'.
this.foo; // number[]
}
}

class C7 {
method1() {
this.foo = 0;
this.foo = this.foo + "abc"; // Error, 'string' not assignable to 'number'
~~~~~~~~
!!! error TS2322: Type 'string' is not assignable to type 'number'.
}
}

// When this.xxx assigmment declarations occur only in methods, an assigned empty array
// literal value is given type 'any[]'.

class C8 {
method1() {
this.bar = []; // Error: Implicit any[]
~~~~~~~~~~~~~
!!! error TS7008: Member 'bar' implicitly has an 'any[]' type.
this.bar.push("baz");
this.bar; // any[]
}
}

200 changes: 200 additions & 0 deletions testdata/baselines/reference/compiler/thisPropertyAssignmentTyping.symbols
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Original file line number Diff line number Diff line change
@@ -0,0 +1,200 @@
//// [tests/cases/compiler/thisPropertyAssignmentTyping.ts] ////

=== main.js ===
class C1 {
>C1 : Symbol(C1, Decl(main.js, 0, 0))

constructor() {
this.foo = [3];
>this.foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
>this : Symbol(C1, Decl(main.js, 0, 0))
>foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))

this.foo = [this.foo[0] * 2];
>this.foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
>this : Symbol(C1, Decl(main.js, 0, 0))
>foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
>this.foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
>this : Symbol(C1, Decl(main.js, 0, 0))
>foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))

this.foo; // number[]
>this.foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
>this : Symbol(C1, Decl(main.js, 0, 0))
>foo : Symbol(C1.foo, Decl(main.js, 1, 19), Decl(main.js, 2, 23))
}
}

class C2 {
>C2 : Symbol(C2, Decl(main.js, 6, 1))

constructor() {
this.foo = [3];
>this.foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
>this : Symbol(C2, Decl(main.js, 6, 1))
>foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))

this.foo; // number[]
>this.foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
>this : Symbol(C2, Decl(main.js, 6, 1))
>foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
}
method1() {
>method1 : Symbol(C2.method1, Decl(main.js, 12, 5))

this.foo = [this.foo[0] * 2];
>this.foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
>this : Symbol(C2, Decl(main.js, 6, 1))
>foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
>this.foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
>this : Symbol(C2, Decl(main.js, 6, 1))
>foo : Symbol(C2.foo, Decl(main.js, 9, 19), Decl(main.js, 13, 15))
}
}

class C4 {
>C4 : Symbol(C4, Decl(main.js, 16, 1))

constructor() {
this.bar = [];
>this.bar : Symbol(C4.bar, Decl(main.js, 19, 19))
>this : Symbol(C4, Decl(main.js, 16, 1))
>bar : Symbol(C4.bar, Decl(main.js, 19, 19))

this.bar.push("baz");
>this.bar.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
>this.bar : Symbol(C4.bar, Decl(main.js, 19, 19))
>this : Symbol(C4, Decl(main.js, 16, 1))
>bar : Symbol(C4.bar, Decl(main.js, 19, 19))
>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))

this.bar; // string[]
>this.bar : Symbol(C4.bar, Decl(main.js, 19, 19))
>this : Symbol(C4, Decl(main.js, 16, 1))
>bar : Symbol(C4.bar, Decl(main.js, 19, 19))
}
}

// When this.xxx assignment declarations occur only in methods, the declared type of the
// property is a union of the types of all assigned values, excluding values from self-referential
// assignments, plus undefined. Local CFA removes the undefined when possible.

class C5 {
>C5 : Symbol(C5, Decl(main.js, 24, 1))

method1() {
>method1 : Symbol(C5.method1, Decl(main.js, 30, 10))

this.foo; // number[] | undefined
>this.foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this : Symbol(C5, Decl(main.js, 24, 1))
>foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))

this.foo = [3]
>this.foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this : Symbol(C5, Decl(main.js, 24, 1))
>foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))

this.foo = [this.foo[0] * 2]
>this.foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this : Symbol(C5, Decl(main.js, 24, 1))
>foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this.foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this : Symbol(C5, Decl(main.js, 24, 1))
>foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))

this.foo; // number[]
>this.foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
>this : Symbol(C5, Decl(main.js, 24, 1))
>foo : Symbol(C5.foo, Decl(main.js, 32, 17), Decl(main.js, 33, 22))
}
}

class C6 {
>C6 : Symbol(C6, Decl(main.js, 37, 1))

method1() {
>method1 : Symbol(C6.method1, Decl(main.js, 39, 10))

this.foo = [3];
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))

this.foo; // number[]
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
}
method2() {
>method2 : Symbol(C6.method2, Decl(main.js, 43, 5))

this.foo; // number[] | undefined
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))

this.foo = [this.foo[0] * 2]; // Error: Object is possibly undefined
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))

this.foo; // number[]
>this.foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
>this : Symbol(C6, Decl(main.js, 37, 1))
>foo : Symbol(C6.foo, Decl(main.js, 40, 15), Decl(main.js, 45, 17))
}
}

class C7 {
>C7 : Symbol(C7, Decl(main.js, 49, 1))

method1() {
>method1 : Symbol(C7.method1, Decl(main.js, 51, 10))

this.foo = 0;
>this.foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))
>this : Symbol(C7, Decl(main.js, 49, 1))
>foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))

this.foo = this.foo + "abc"; // Error, 'string' not assignable to 'number'
>this.foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))
>this : Symbol(C7, Decl(main.js, 49, 1))
>foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))
>this.foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))
>this : Symbol(C7, Decl(main.js, 49, 1))
>foo : Symbol(C7.foo, Decl(main.js, 52, 15), Decl(main.js, 53, 21))
}
}

// When this.xxx assigmment declarations occur only in methods, an assigned empty array
// literal value is given type 'any[]'.

class C8 {
>C8 : Symbol(C8, Decl(main.js, 56, 1))

method1() {
>method1 : Symbol(C8.method1, Decl(main.js, 61, 10))

this.bar = []; // Error: Implicit any[]
>this.bar : Symbol(C8.bar, Decl(main.js, 62, 15))
>this : Symbol(C8, Decl(main.js, 56, 1))
>bar : Symbol(C8.bar, Decl(main.js, 62, 15))

this.bar.push("baz");
>this.bar.push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))
>this.bar : Symbol(C8.bar, Decl(main.js, 62, 15))
>this : Symbol(C8, Decl(main.js, 56, 1))
>bar : Symbol(C8.bar, Decl(main.js, 62, 15))
>push : Symbol(Array.push, Decl(lib.es5.d.ts, --, --))

this.bar; // any[]
>this.bar : Symbol(C8.bar, Decl(main.js, 62, 15))
>this : Symbol(C8, Decl(main.js, 56, 1))
>bar : Symbol(C8.bar, Decl(main.js, 62, 15))
}
}

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