Support typeof and instanceof

Author: ahejlsberg

src/compiler/binder.ts

@@ -856,9 +856,8 @@ namespace ts {
         function isNarrowableReference(expr: Expression): boolean {
             return expr.kind === SyntaxKind.Identifier || expr.kind === SyntaxKind.ThisKeyword || expr.kind === SyntaxKind.SuperKeyword ||
                 (isPropertyAccessExpression(expr) || isNonNullExpression(expr) || isParenthesizedExpression(expr)) && isNarrowableReference(expr.expression) ||
-                isElementAccessExpression(expr) &&
-                isStringOrNumericLiteralLike(expr.argumentExpression) &&
-                isNarrowableReference(expr.expression);
+                isElementAccessExpression(expr) && isStringOrNumericLiteralLike(expr.argumentExpression) && isNarrowableReference(expr.expression) ||
+                isOptionalChain(expr);
         }
 
         function hasNarrowableArgument(expr: CallExpression) {

src/compiler/checker.ts

@@ -19360,7 +19360,7 @@ namespace ts {
                         if (isMatchingReference(reference, right)) {
                             return narrowTypeByEquality(type, operator, left, assumeTrue);
                         }
-                        if (assumeTrue && (operator === SyntaxKind.EqualsEqualsEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken)) {
+                        if (assumeTrue && strictNullChecks) {
                             if (optionalChainContainsReference(left, reference)) {
                                 type = narrowTypeByOptionalChainContainment(type, operator, right);
                             }
@@ -19397,7 +19397,9 @@ namespace ts {
                 // the type of obj if (a) the operator is === and the type of value doesn't include undefined or (b) the
                 // operator is !== and the type of value is undefined.
                 const valueType = getTypeOfExpression(value);
-                return operator === SyntaxKind.EqualsEqualsEqualsToken && !(getTypeFacts(valueType) & TypeFacts.EQUndefined) ||
+                return operator === SyntaxKind.EqualsEqualsToken && !(getTypeFacts(valueType) & TypeFacts.EQUndefinedOrNull) ||
+                    operator === SyntaxKind.EqualsEqualsEqualsToken && !(getTypeFacts(valueType) & TypeFacts.EQUndefined) ||
+                    operator === SyntaxKind.ExclamationEqualsToken && valueType.flags & TypeFlags.Nullable ||
                     operator === SyntaxKind.ExclamationEqualsEqualsToken && valueType.flags & TypeFlags.Undefined ?
                     getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull) : type;
             }
@@ -19452,6 +19454,10 @@ namespace ts {
                 // We have '==', '!=', '===', or !==' operator with 'typeof xxx' and string literal operands
                 const target = getReferenceCandidate(typeOfExpr.expression);
                 if (!isMatchingReference(reference, target)) {
+                    if (assumeTrue && (operator === SyntaxKind.EqualsEqualsToken || operator === SyntaxKind.EqualsEqualsEqualsToken) &&
+                        strictNullChecks && optionalChainContainsReference(target, reference)) {
+                        return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull);
+                    }
                     // For a reference of the form 'x.y', a 'typeof x === ...' type guard resets the
                     // narrowed type of 'y' to its declared type.
                     if (containsMatchingReference(reference, target)) {
@@ -19633,6 +19639,9 @@ namespace ts {
             function narrowTypeByInstanceof(type: Type, expr: BinaryExpression, assumeTrue: boolean): Type {
                 const left = getReferenceCandidate(expr.left);
                 if (!isMatchingReference(reference, left)) {
+                    if (assumeTrue && strictNullChecks && optionalChainContainsReference(left, reference)) {
+                        return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull);
+                    }
                     // For a reference of the form 'x.y', an 'x instanceof T' type guard resets the
                     // narrowed type of 'y' to its declared type. We do this because preceding 'x.y'
                     // references might reference a different 'y' property. However, we make an exception