nullish coalescing commit

src/compiler/binder.ts

@@ -1015,7 +1015,8 @@ namespace ts {
                 else {
                     return node.kind === SyntaxKind.BinaryExpression && (
                         (<BinaryExpression>node).operatorToken.kind === SyntaxKind.AmpersandAmpersandToken ||
-                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken);
+                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken ||
+                        (<BinaryExpression>node).operatorToken.kind === SyntaxKind.QuestionQuestionToken);
                 }
             }
         }
@@ -1466,7 +1467,7 @@ namespace ts {
 
         function bindBinaryExpressionFlow(node: BinaryExpression) {
             const operator = node.operatorToken.kind;
-            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken) {
+            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken || operator === SyntaxKind.QuestionQuestionToken) {
                 if (isTopLevelLogicalExpression(node)) {
                     const postExpressionLabel = createBranchLabel();
                     bindLogicalExpression(node, postExpressionLabel, postExpressionLabel);
@@ -2948,7 +2949,7 @@ namespace ts {
             init = init && getRightMostAssignedExpression(init);
             if (init) {
                 const isPrototypeAssignment = isPrototypeAccess(isVariableDeclaration(node) ? node.name : isBinaryExpression(node) ? node.left : node);
-                return !!getExpandoInitializer(isBinaryExpression(init) && init.operatorToken.kind === SyntaxKind.BarBarToken ? init.right : init, isPrototypeAssignment);
+                return !!getExpandoInitializer(isBinaryExpression(init) && (init.operatorToken.kind === SyntaxKind.BarBarToken || init.operatorToken.kind === SyntaxKind.QuestionQuestionToken) ? init.right : init, isPrototypeAssignment);
             }
             return false;
         }
@@ -3424,7 +3425,10 @@ namespace ts {
         const operatorTokenKind = node.operatorToken.kind;
         const leftKind = node.left.kind;
 
-        if (operatorTokenKind === SyntaxKind.EqualsToken && leftKind === SyntaxKind.ObjectLiteralExpression) {
+        if (operatorTokenKind === SyntaxKind.QuestionQuestionToken) {
+            transformFlags |= TransformFlags.AssertESNext;
+        }
+        else if (operatorTokenKind === SyntaxKind.EqualsToken && leftKind === SyntaxKind.ObjectLiteralExpression) {
             // Destructuring object assignments with are ES2015 syntax
             // and possibly ES2018 if they contain rest
             transformFlags |= TransformFlags.AssertES2018 | TransformFlags.AssertES2015 | TransformFlags.AssertDestructuringAssignment;

src/compiler/checker.ts

@@ -13254,7 +13254,7 @@ namespace ts {
                     return isContextSensitive((<ConditionalExpression>node).whenTrue) ||
                         isContextSensitive((<ConditionalExpression>node).whenFalse);
                 case SyntaxKind.BinaryExpression:
-                    return (<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken &&
+                    return ((<BinaryExpression>node).operatorToken.kind === SyntaxKind.BarBarToken || (<BinaryExpression>node).operatorToken.kind === SyntaxKind.QuestionQuestionToken) &&
                         (isContextSensitive((<BinaryExpression>node).left) || isContextSensitive((<BinaryExpression>node).right));
                 case SyntaxKind.PropertyAssignment:
                     return isContextSensitive((<PropertyAssignment>node).initializer);
@@ -19697,7 +19697,8 @@ namespace ts {
             // will be a subtype or the same type as the argument.
             function narrowType(type: Type, expr: Expression, assumeTrue: boolean): Type {
                 // for `a?.b`, we emulate a synthetic `a !== null && a !== undefined` condition for `a`
-                if (isOptionalChainRoot(expr.parent)) {
+                if (isOptionalChainRoot(expr.parent) ||
+                    isBinaryExpression(expr.parent) && expr.parent.operatorToken.kind === SyntaxKind.QuestionQuestionToken && expr.parent.left === expr) {
                     return narrowTypeByOptionality(type, expr, assumeTrue);
                 }
                 switch (expr.kind) {
@@ -20903,6 +20904,7 @@ namespace ts {
                     }
                     return contextSensitive === true ? getTypeOfExpression(left) : contextSensitive;
                 case SyntaxKind.BarBarToken:
+                case SyntaxKind.QuestionQuestionToken:
                     // When an || expression has a contextual type, the operands are contextually typed by that type, except
                     // when that type originates in a binding pattern, the right operand is contextually typed by the type of
                     // the left operand. When an || expression has no contextual type, the right operand is contextually typed
@@ -26467,16 +26469,29 @@ namespace ts {
             if (isInJSFile(node) && getAssignedExpandoInitializer(node)) {
                 return checkExpression(node.right, checkMode);
             }
+            checkGrammarNullishCoalesceWithLogicalExpression(node);
             return checkBinaryLikeExpression(node.left, node.operatorToken, node.right, checkMode, node);
         }
 
+        function checkGrammarNullishCoalesceWithLogicalExpression(node: BinaryExpression) {
+            const { left, operatorToken, right } = node;
+            if (operatorToken.kind === SyntaxKind.QuestionQuestionToken) {
+                if (isBinaryExpression(left) && (left.operatorToken.kind === SyntaxKind.BarBarToken || left.operatorToken.kind === SyntaxKind.AmpersandAmpersandToken)) {
+                    grammarErrorOnNode(left, Diagnostics._0_and_1_operations_cannot_be_mixed_without_parentheses, tokenToString(left.operatorToken.kind), tokenToString(operatorToken.kind));
+                }
+                if (isBinaryExpression(right) && (right.operatorToken.kind === SyntaxKind.BarBarToken || right.operatorToken.kind === SyntaxKind.AmpersandAmpersandToken)) {
+                    grammarErrorOnNode(right, Diagnostics._0_and_1_operations_cannot_be_mixed_without_parentheses, tokenToString(right.operatorToken.kind), tokenToString(operatorToken.kind));
+                }
+            }
+        }
+
         function checkBinaryLikeExpression(left: Expression, operatorToken: Node, right: Expression, checkMode?: CheckMode, errorNode?: Node): Type {
             const operator = operatorToken.kind;
             if (operator === SyntaxKind.EqualsToken && (left.kind === SyntaxKind.ObjectLiteralExpression || left.kind === SyntaxKind.ArrayLiteralExpression)) {
                 return checkDestructuringAssignment(left, checkExpression(right, checkMode), checkMode, right.kind === SyntaxKind.ThisKeyword);
             }
             let leftType: Type;
-            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken) {
+            if (operator === SyntaxKind.AmpersandAmpersandToken || operator === SyntaxKind.BarBarToken || operator === SyntaxKind.QuestionQuestionToken) {
                 leftType = checkTruthinessExpression(left, checkMode);
             }
             else {
@@ -26637,6 +26652,10 @@ namespace ts {
                     return getTypeFacts(leftType) & TypeFacts.Falsy ?
                         getUnionType([removeDefinitelyFalsyTypes(leftType), rightType], UnionReduction.Subtype) :
                         leftType;
+                case SyntaxKind.QuestionQuestionToken:
+                    return getTypeFacts(leftType) & TypeFacts.EQUndefinedOrNull ?
+                        getUnionType([getNonNullableType(leftType), rightType], UnionReduction.Subtype) :
+                        leftType;
                 case SyntaxKind.EqualsToken:
                     const declKind = isBinaryExpression(left.parent) ? getAssignmentDeclarationKind(left.parent) : AssignmentDeclarationKind.None;
                     checkAssignmentDeclaration(declKind, rightType);

src/compiler/diagnosticMessages.json

@@ -3273,6 +3273,10 @@
         "category": "Error",
         "code": 5075
     },
+    "'{0}' and '{1}' operations cannot be mixed without parentheses.": {
+        "category": "Error",
+        "code": 5076
+    },
 
     "Generates a sourcemap for each corresponding '.d.ts' file.": {
         "category": "Message",

src/compiler/factory.ts

@@ -3248,6 +3248,10 @@ namespace ts {
         return createBinary(left, SyntaxKind.BarBarToken, right);
     }
 
+    export function createNullishCoalesce(left: Expression, right: Expression) {
+        return createBinary(left, SyntaxKind.QuestionQuestionToken, right);
+    }
+
     export function createLogicalNot(operand: Expression) {
         return createPrefix(SyntaxKind.ExclamationToken, operand);
     }
@@ -4593,7 +4597,7 @@ namespace ts {
         const binaryOperatorPrecedence = getOperatorPrecedence(SyntaxKind.BinaryExpression, binaryOperator);
         const binaryOperatorAssociativity = getOperatorAssociativity(SyntaxKind.BinaryExpression, binaryOperator);
         const emittedOperand = skipPartiallyEmittedExpressions(operand);
-        if (!isLeftSideOfBinary && operand.kind === SyntaxKind.ArrowFunction && binaryOperatorPrecedence > 4) {
+        if (!isLeftSideOfBinary && operand.kind === SyntaxKind.ArrowFunction && binaryOperatorPrecedence > 3) {
             // We need to parenthesize arrow functions on the right side to avoid it being
             // parsed as parenthesized expression: `a && (() => {})`
             return true;

src/compiler/parser.ts

@@ -3009,6 +3009,10 @@ namespace ts {
                     return parseJSDocAllType(/*postfixEquals*/ false);
                 case SyntaxKind.AsteriskEqualsToken:
                     return parseJSDocAllType(/*postfixEquals*/ true);
+                case SyntaxKind.QuestionQuestionToken:
+                    // If there is '??', consider that is prefix '?' in JSDoc type.
+                    scanner.reScanQuestionToken();
+                    // falls through
                 case SyntaxKind.QuestionToken:
                     return parseJSDocUnknownOrNullableType();
                 case SyntaxKind.FunctionKeyword:
@@ -4800,6 +4804,7 @@ namespace ts {
                 case SyntaxKind.ExclamationEqualsEqualsToken:   // foo<x> !==
                 case SyntaxKind.AmpersandAmpersandToken:        // foo<x> &&
                 case SyntaxKind.BarBarToken:                    // foo<x> ||
+                case SyntaxKind.QuestionQuestionToken:          // foo<x> ??
                 case SyntaxKind.CaretToken:                     // foo<x> ^
                 case SyntaxKind.AmpersandToken:                 // foo<x> &
                 case SyntaxKind.BarToken:                       // foo<x> |

src/compiler/scanner.ts

@@ -33,6 +33,7 @@ namespace ts {
         scanJsxAttributeValue(): SyntaxKind;
         reScanJsxToken(): JsxTokenSyntaxKind;
         reScanLessThanToken(): SyntaxKind;
+        reScanQuestionToken(): SyntaxKind;
         scanJsxToken(): JsxTokenSyntaxKind;
         scanJsDocToken(): JSDocSyntaxKind;
         scan(): SyntaxKind;
@@ -184,6 +185,7 @@ namespace ts {
         "&&": SyntaxKind.AmpersandAmpersandToken,
         "||": SyntaxKind.BarBarToken,
         "?": SyntaxKind.QuestionToken,
+        "??": SyntaxKind.QuestionQuestionToken,
         "?.": SyntaxKind.QuestionDotToken,
         ":": SyntaxKind.ColonToken,
         "=": SyntaxKind.EqualsToken,
@@ -902,6 +904,7 @@ namespace ts {
             scanJsxAttributeValue,
             reScanJsxToken,
             reScanLessThanToken,
+            reScanQuestionToken,
             scanJsxToken,
             scanJsDocToken,
             scan,
@@ -1834,6 +1837,10 @@ namespace ts {
                             pos++;
                             return token = SyntaxKind.QuestionDotToken;
                         }
+                        if (text.charCodeAt(pos) === CharacterCodes.question) {
+                            pos++;
+                            return token = SyntaxKind.QuestionQuestionToken;
+                        }
                         return token = SyntaxKind.QuestionToken;
                     case CharacterCodes.openBracket:
                         pos++;
@@ -2023,6 +2030,12 @@ namespace ts {
             return token;
         }
 
+        function reScanQuestionToken(): SyntaxKind {
+            Debug.assert(token === SyntaxKind.QuestionQuestionToken, "'reScanQuestionToken' should only be called on a '??'");
+            pos = tokenPos + 1;
+            return token = SyntaxKind.QuestionToken;
+        }
+
         function scanJsxToken(): JsxTokenSyntaxKind {
             startPos = tokenPos = pos;
 

src/compiler/transformers/esnext.ts

@@ -2,7 +2,7 @@
 namespace ts {
     export function transformESNext(context: TransformationContext) {
         const {
-            hoistVariableDeclaration
+            hoistVariableDeclaration,
         } = context;
 
         return chainBundle(transformSourceFile);
@@ -28,7 +28,12 @@ namespace ts {
                         Debug.assertNotNode(updated, isSyntheticReference);
                         return updated;
                     }
-                    // falls through
+                    return visitEachChild(node, visitor, context);
+                case SyntaxKind.BinaryExpression:
+                    if ((<BinaryExpression>node).operatorToken.kind === SyntaxKind.QuestionQuestionToken) {
+                        return transformNullishCoalescingExpression(<BinaryExpression>node);
+                    }
+                    return visitEachChild(node, visitor, context);
                 default:
                     return visitEachChild(node, visitor, context);
             }
@@ -172,5 +177,39 @@ namespace ts {
             );
             return thisArg ? createSyntheticReferenceExpression(target, thisArg) : target;
         }
+
+        function createNotNullCondition(node: Expression) {
+            return createBinary(
+                createBinary(
+                    node,
+                    createToken(SyntaxKind.ExclamationEqualsEqualsToken),
+                    createNull()
+                ),
+                createToken(SyntaxKind.AmpersandAmpersandToken),
+                createBinary(
+                    node,
+                    createToken(SyntaxKind.ExclamationEqualsEqualsToken),
+                    createVoidZero()
+                )
+            );
+        }
+
+        function transformNullishCoalescingExpression(node: BinaryExpression) {
+            const expressions: Expression[] = [];
+            let left = visitNode(node.left, visitor, isExpression);
+            if (!isIdentifier(left)) {
+                const temp = createTempVariable(hoistVariableDeclaration);
+                expressions.push(createAssignment(temp, left));
+                left = temp;
+            }
+            expressions.push(
+                createParen(
+                    createConditional(
+                        createNotNullCondition(left),
+                        left,
+                        visitNode(node.right, visitor, isExpression)))
+                );
+            return inlineExpressions(expressions);
+        }
     }
 }

src/compiler/types.ts

@@ -184,6 +184,7 @@ namespace ts {
         QuestionToken,
         ColonToken,
         AtToken,
+        QuestionQuestionToken,
         /** Only the JSDoc scanner produces BacktickToken. The normal scanner produces NoSubstitutionTemplateLiteral and related kinds. */
         BacktickToken,
         // Assignments
@@ -1566,7 +1567,8 @@ namespace ts {
 
     // see: https://tc39.github.io/ecma262/#prod-AssignmentExpression
     export type AssignmentOperatorOrHigher
-        = LogicalOperatorOrHigher
+        = SyntaxKind.QuestionQuestionToken
+        | LogicalOperatorOrHigher
         | AssignmentOperator
         ;