microsoft · rbuckton · Sep 9, 2016 · Sep 9, 2016 · DanielRosenwasser · Sep 9, 2016
diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
@@ -1150,8 +1150,8 @@ namespace ts {
             currentFlow = finishFlowLabel(postExpressionLabel);
         }
 
-        function bindInitializedVariableFlow(node: VariableDeclaration | BindingElement) {
-            const name = node.name;
+        function bindInitializedVariableFlow(node: VariableDeclaration | ArrayBindingElement) {
+            const name = !isOmittedExpression(node) ? node.name : undefined;
             if (isBindingPattern(name)) {
                 for (const child of name.elements) {
                     bindInitializedVariableFlow(child);

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -2505,8 +2505,8 @@ namespace ts {
                 }
             }
 
-            function buildBindingElementDisplay(bindingElement: BindingElement, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
-                if (bindingElement.kind === SyntaxKind.OmittedExpression) {
+            function buildBindingElementDisplay(bindingElement: ArrayBindingElement, writer: SymbolWriter, enclosingDeclaration?: Node, flags?: TypeFormatFlags, symbolStack?: Symbol[]) {
+                if (isOmittedExpression(bindingElement)) {
                     return;
                 }
                 Debug.assert(bindingElement.kind === SyntaxKind.BindingElement);
@@ -3125,7 +3125,7 @@ namespace ts {
         }
 
         // Return the type implied by an object binding pattern
-        function getTypeFromObjectBindingPattern(pattern: BindingPattern, includePatternInType: boolean, reportErrors: boolean): Type {
+        function getTypeFromObjectBindingPattern(pattern: ObjectBindingPattern, includePatternInType: boolean, reportErrors: boolean): Type {
             const members = createMap<Symbol>();
             let hasComputedProperties = false;
             forEach(pattern.elements, e => {
@@ -3156,11 +3156,12 @@ namespace ts {
         // Return the type implied by an array binding pattern
         function getTypeFromArrayBindingPattern(pattern: BindingPattern, includePatternInType: boolean, reportErrors: boolean): Type {
             const elements = pattern.elements;
-            if (elements.length === 0 || elements[elements.length - 1].dotDotDotToken) {
+            const lastElement = lastOrUndefined(elements);
+            if (elements.length === 0 || (!isOmittedExpression(lastElement) && lastElement.dotDotDotToken)) {
                 return languageVersion >= ScriptTarget.ES6 ? createIterableType(anyType) : anyArrayType;
             }
             // If the pattern has at least one element, and no rest element, then it should imply a tuple type.
-            const elementTypes = map(elements, e => e.kind === SyntaxKind.OmittedExpression ? anyType : getTypeFromBindingElement(e, includePatternInType, reportErrors));
+            const elementTypes = map(elements, e => isOmittedExpression(e) ? anyType : getTypeFromBindingElement(e, includePatternInType, reportErrors));
             let result = createTupleType(elementTypes);
             if (includePatternInType) {
                 result = cloneTypeReference(result);
@@ -3178,8 +3179,8 @@ namespace ts {
         // the parameter.
         function getTypeFromBindingPattern(pattern: BindingPattern, includePatternInType?: boolean, reportErrors?: boolean): Type {
             return pattern.kind === SyntaxKind.ObjectBindingPattern
-                ? getTypeFromObjectBindingPattern(pattern, includePatternInType, reportErrors)
-                : getTypeFromArrayBindingPattern(pattern, includePatternInType, reportErrors);
+                ? getTypeFromObjectBindingPattern(<ObjectBindingPattern>pattern, includePatternInType, reportErrors)
+                : getTypeFromArrayBindingPattern(<ArrayBindingPattern>pattern, includePatternInType, reportErrors);
         }
 
         // Return the type associated with a variable, parameter, or property declaration. In the simple case this is the type
@@ -12413,7 +12414,7 @@ namespace ts {
         function assignBindingElementTypes(node: VariableLikeDeclaration) {
             if (isBindingPattern(node.name)) {
                 for (const element of (<BindingPattern>node.name).elements) {
-                    if (element.kind !== SyntaxKind.OmittedExpression) {
+                    if (!isOmittedExpression(element)) {
                         if (element.name.kind === SyntaxKind.Identifier) {
                             getSymbolLinks(getSymbolOfNode(element)).type = getTypeForBindingElement(element);
                         }
@@ -13889,7 +13890,12 @@ namespace ts {
             pattern: BindingPattern,
             predicateVariableNode: Node,
             predicateVariableName: string) {
-            for (const { name } of pattern.elements) {
+            for (const element of pattern.elements) {
+                if (isOmittedExpression(element)) {
+                    continue;
+                }
+
+                const name = element.name;
                 if (name.kind === SyntaxKind.Identifier &&
                     (<Identifier>name).text === predicateVariableName) {
                     error(predicateVariableNode,
@@ -19999,7 +20005,7 @@ namespace ts {
             else {
                 const elements = (<BindingPattern>name).elements;
                 for (const element of elements) {
-                    if (element.kind !== SyntaxKind.OmittedExpression) {
+                    if (!isOmittedExpression(element)) {
                         checkGrammarNameInLetOrConstDeclarations(element.name);
                     }
                 }

diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
@@ -554,9 +554,9 @@ const _super = (function (geti, seti) {
 
                 // Binding patterns
                 case SyntaxKind.ObjectBindingPattern:
-                    return emitObjectBindingPattern(<BindingPattern>node);
+                    return emitObjectBindingPattern(<ObjectBindingPattern>node);
                 case SyntaxKind.ArrayBindingPattern:
-                    return emitArrayBindingPattern(<BindingPattern>node);
+                    return emitArrayBindingPattern(<ArrayBindingPattern>node);
                 case SyntaxKind.BindingElement:
                     return emitBindingElement(<BindingElement>node);
 

diff --git a/src/compiler/factory.ts b/src/compiler/factory.ts
@@ -368,13 +368,13 @@ namespace ts {
         return node;
     }
 
-    export function createArrayBindingPattern(elements: BindingElement[], location?: TextRange) {
+    export function createArrayBindingPattern(elements: ArrayBindingElement[], location?: TextRange) {
         const node = <ArrayBindingPattern>createNode(SyntaxKind.ArrayBindingPattern, location);
         node.elements = createNodeArray(elements);
         return node;
     }
 
-    export function updateArrayBindingPattern(node: ArrayBindingPattern, elements: BindingElement[]) {
+    export function updateArrayBindingPattern(node: ArrayBindingPattern, elements: ArrayBindingElement[]) {
         if (node.elements !== elements) {
             return updateNode(createArrayBindingPattern(elements, node), node);
         }

diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
@@ -4815,9 +4815,9 @@ namespace ts {
 
         // DECLARATIONS
 
-        function parseArrayBindingElement(): BindingElement {
+        function parseArrayBindingElement(): ArrayBindingElement {
             if (token() === SyntaxKind.CommaToken) {
-                return <BindingElement>createNode(SyntaxKind.OmittedExpression);
+                return <OmittedExpression>createNode(SyntaxKind.OmittedExpression);
             }
             const node = <BindingElement>createNode(SyntaxKind.BindingElement);
             node.dotDotDotToken = parseOptionalToken(SyntaxKind.DotDotDotToken);
@@ -4842,16 +4842,16 @@ namespace ts {
             return finishNode(node);
         }
 
-        function parseObjectBindingPattern(): BindingPattern {
-            const node = <BindingPattern>createNode(SyntaxKind.ObjectBindingPattern);
+        function parseObjectBindingPattern(): ObjectBindingPattern {
+            const node = <ObjectBindingPattern>createNode(SyntaxKind.ObjectBindingPattern);
             parseExpected(SyntaxKind.OpenBraceToken);
             node.elements = parseDelimitedList(ParsingContext.ObjectBindingElements, parseObjectBindingElement);
             parseExpected(SyntaxKind.CloseBraceToken);
             return finishNode(node);
         }
 
-        function parseArrayBindingPattern(): BindingPattern {
-            const node = <BindingPattern>createNode(SyntaxKind.ArrayBindingPattern);
+        function parseArrayBindingPattern(): ArrayBindingPattern {
+            const node = <ArrayBindingPattern>createNode(SyntaxKind.ArrayBindingPattern);
             parseExpected(SyntaxKind.OpenBracketToken);
             node.elements = parseDelimitedList(ParsingContext.ArrayBindingElements, parseArrayBindingElement);
             parseExpected(SyntaxKind.CloseBracketToken);

diff --git a/src/compiler/transformers/destructuring.ts b/src/compiler/transformers/destructuring.ts
@@ -326,12 +326,15 @@ namespace ts {
                 }
                 for (let i = 0; i < numElements; i++) {
                     const element = elements[i];
-                    if (name.kind === SyntaxKind.ObjectBindingPattern) {
+                    if (isOmittedExpression(element)) {
+                        continue;
+                    }
+                    else if (name.kind === SyntaxKind.ObjectBindingPattern) {
                         // Rewrite element to a declaration with an initializer that fetches property
                         const propName = element.propertyName || <Identifier>element.name;
                         emitBindingElement(element, createDestructuringPropertyAccess(value, propName));
                     }
-                    else if (element.kind !== SyntaxKind.OmittedExpression) {
+                    else {
                         if (!element.dotDotDotToken) {
                             // Rewrite element to a declaration that accesses array element at index i
                             emitBindingElement(element, createElementAccess(value, i));

diff --git a/src/compiler/transformers/es6.ts b/src/compiler/transformers/es6.ts
@@ -1945,7 +1945,9 @@ namespace ts {
                 }
                 else {
                     for (const element of (<BindingPattern>node).elements) {
-                        visit(element.name);
+                        if (!isOmittedExpression(element)) {
+                            visit(element.name);
+                        }
                     }
                 }
             }
@@ -2289,7 +2291,9 @@ namespace ts {
             const name = decl.name;
             if (isBindingPattern(name)) {
                 for (const element of name.elements) {
-                    processLoopVariableDeclaration(element, loopParameters, loopOutParameters);
+                    if (!isOmittedExpression(element)) {
+                        processLoopVariableDeclaration(element, loopParameters, loopOutParameters);
+                    }
                 }
             }
             else {

diff --git a/src/compiler/transformers/module/module.ts b/src/compiler/transformers/module/module.ts
@@ -660,7 +660,9 @@ namespace ts {
         function addExportMemberAssignmentsForBindingName(resultStatements: Statement[], name: BindingName): void {
             if (isBindingPattern(name)) {
                 for (const element of name.elements) {
-                    addExportMemberAssignmentsForBindingName(resultStatements, element.name);
+                    if (!isOmittedExpression(element)) {
+                        addExportMemberAssignmentsForBindingName(resultStatements, element.name);
+                    }
                 }
             }
             else {

diff --git a/src/compiler/transformers/module/system.ts b/src/compiler/transformers/module/system.ts
@@ -1368,7 +1368,11 @@ namespace ts {
             exportedFunctionDeclarations.push(createDeclarationExport(node));
         }
 
-        function hoistBindingElement(node: VariableDeclaration | BindingElement, isExported: boolean): void {
+        function hoistBindingElement(node: VariableDeclaration | ArrayBindingElement, isExported: boolean): void {
+            if (isOmittedExpression(node)) {
+                return;
+            }
+
             const name = node.name;
             if (isIdentifier(name)) {
                 hoistVariableDeclaration(getSynthesizedClone(name));
@@ -1381,11 +1385,11 @@ namespace ts {
             }
         }
 
-        function hoistExportedBindingElement(node: VariableDeclaration | BindingElement) {
+        function hoistExportedBindingElement(node: VariableDeclaration | ArrayBindingElement) {
             hoistBindingElement(node, /*isExported*/ true);
         }
 
-        function hoistNonExportedBindingElement(node: VariableDeclaration | BindingElement) {
+        function hoistNonExportedBindingElement(node: VariableDeclaration | ArrayBindingElement) {
             hoistBindingElement(node, /*isExported*/ false);
         }
 

diff --git a/src/compiler/transformers/ts.ts b/src/compiler/transformers/ts.ts
@@ -2187,7 +2187,7 @@ namespace ts {
          *
          * @param node The function expression node.
          */
-        function visitFunctionExpression(node: FunctionExpression) {
+        function visitFunctionExpression(node: FunctionExpression): Expression {
             if (nodeIsMissing(node.body)) {
                 return createOmittedExpression();
             }

diff --git a/src/compiler/types.ts b/src/compiler/types.ts
@@ -688,14 +688,20 @@ namespace ts {
     }
 
     export interface BindingPattern extends Node {
-        elements: NodeArray<BindingElement>;
+        elements: NodeArray<BindingElement | ArrayBindingElement>;
     }
 
     // @kind(SyntaxKind.ObjectBindingPattern)
-    export interface ObjectBindingPattern extends BindingPattern { }
+    export interface ObjectBindingPattern extends BindingPattern {
+        elements: NodeArray<BindingElement>;
+    }
+
+    export type ArrayBindingElement = BindingElement | OmittedExpression;
 
     // @kind(SyntaxKind.ArrayBindingPattern)
-    export interface ArrayBindingPattern extends BindingPattern { }
+    export interface ArrayBindingPattern extends BindingPattern {
+        elements: NodeArray<ArrayBindingElement>;
+    }
 
     /**
      * Several node kinds share function-like features such as a signature,
@@ -868,7 +874,9 @@ namespace ts {
     }
 
     // @kind(SyntaxKind.OmittedExpression)
-    export interface OmittedExpression extends Expression { }
+    export interface OmittedExpression extends Expression {
+        _omittedExpressionBrand: any;
+    }
 
     // Represents an expression that is elided as part of a transformation to emit comments on a
     // not-emitted node. The 'expression' property of a NotEmittedExpression should be emitted.

diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
@@ -3701,6 +3701,12 @@ namespace ts {
         return node.kind === SyntaxKind.BindingElement;
     }
 
+    export function isArrayBindingElement(node: Node): node is ArrayBindingElement {
+        const kind = node.kind;
+        return kind === SyntaxKind.BindingElement
+            || kind === SyntaxKind.OmittedExpression;
+    }
+
     // Expression
 
     export function isPropertyAccessExpression(node: Node): node is PropertyAccessExpression {
@@ -3817,6 +3823,10 @@ namespace ts {
             || isPartiallyEmittedExpression(node);
     }
 
+    export function isOmittedExpression(node: Node): node is OmittedExpression {
+        return node.kind === SyntaxKind.OmittedExpression;
+    }
+
     // Misc
 
     export function isTemplateSpan(node: Node): node is TemplateSpan {

diff --git a/src/compiler/visitor.ts b/src/compiler/visitor.ts
@@ -757,7 +757,7 @@ namespace ts {
 
             case SyntaxKind.ArrayBindingPattern:
                 return updateArrayBindingPattern(<ArrayBindingPattern>node,
-                    visitNodes((<ArrayBindingPattern>node).elements, visitor, isBindingElement));
+                    visitNodes((<ArrayBindingPattern>node).elements, visitor, isArrayBindingElement));
 
             case SyntaxKind.BindingElement:
                 return updateBindingElement(<BindingElement>node,

diff --git a/src/services/completions.ts b/src/services/completions.ts
@@ -1192,7 +1192,7 @@ namespace ts.Completions {
                     }
                     if (canGetType) {
                         typeForObject = typeChecker.getTypeAtLocation(objectLikeContainer);
-                        existingMembers = (<BindingPattern>objectLikeContainer).elements;
+                        existingMembers = (<ObjectBindingPattern>objectLikeContainer).elements;
                     }
                 }
                 else {

diff --git a/tests/baselines/reference/exportArrayBindingPattern.js b/tests/baselines/reference/exportArrayBindingPattern.js
@@ -0,0 +1,11 @@
+//// [exportArrayBindingPattern.ts]
+// issue: https://github.com/Microsoft/TypeScript/issues/10778
+const [a, , b] = [1, 2, 3];
+export { a, b };
+
+//// [exportArrayBindingPattern.js]
+"use strict";
+// issue: https://github.com/Microsoft/TypeScript/issues/10778
+var _a = [1, 2, 3], a = _a[0], b = _a[2];
+exports.a = a;
+exports.b = b;
diff --git a/tests/baselines/reference/exportArrayBindingPattern.symbols b/tests/baselines/reference/exportArrayBindingPattern.symbols
@@ -0,0 +1,10 @@
+=== tests/cases/compiler/exportArrayBindingPattern.ts ===
+// issue: https://github.com/Microsoft/TypeScript/issues/10778
+const [a, , b] = [1, 2, 3];
+>a : Symbol(a, Decl(exportArrayBindingPattern.ts, 1, 7))
+>b : Symbol(b, Decl(exportArrayBindingPattern.ts, 1, 11))
+
+export { a, b };
+>a : Symbol(a, Decl(exportArrayBindingPattern.ts, 2, 8))
+>b : Symbol(b, Decl(exportArrayBindingPattern.ts, 2, 11))
+
diff --git a/tests/baselines/reference/exportArrayBindingPattern.types b/tests/baselines/reference/exportArrayBindingPattern.types
@@ -0,0 +1,15 @@
+=== tests/cases/compiler/exportArrayBindingPattern.ts ===
+// issue: https://github.com/Microsoft/TypeScript/issues/10778
+const [a, , b] = [1, 2, 3];
+>a : number
+> : undefined
+>b : number
+>[1, 2, 3] : [number, number, number]
+>1 : number
+>2 : number
+>3 : number
+
+export { a, b };
+>a : number
+>b : number
+
diff --git a/tests/cases/compiler/exportArrayBindingPattern.ts b/tests/cases/compiler/exportArrayBindingPattern.ts
@@ -0,0 +1,4 @@
+// @module: commonjs
+// issue: https://github.com/Microsoft/TypeScript/issues/10778
+const [a, , b] = [1, 2, 3];
+export { a, b };