Function narrowType

test/fixtures/snapshot/typescript.js:71345–71387

(type, expr, assumeTrue)

            // Narrow the given type based on the given expression having the assumed boolean value. The returned type
            // will be a subtype or the same type as the argument.
            function narrowType(type, expr, assumeTrue) {
                // for `a?.b`, we emulate a synthetic `a !== null && a !== undefined` condition for `a`
                if (ts.isExpressionOfOptionalChainRoot(expr) ||
                    ts.isBinaryExpression(expr.parent) && expr.parent.operatorToken.kind === 60 /* SyntaxKind.QuestionQuestionToken */ && expr.parent.left === expr) {
                    return narrowTypeByOptionality(type, expr, assumeTrue);
                }
                switch (expr.kind) {
                    case 79 /* SyntaxKind.Identifier */:
                        // When narrowing a reference to a const variable, non-assigned parameter, or readonly property, we inline
                        // up to five levels of aliased conditional expressions that are themselves declared as const variables.
                        if (!isMatchingReference(reference, expr) && inlineLevel < 5) {
                            var symbol = getResolvedSymbol(expr);
                            if (isConstVariable(symbol)) {
                                var declaration = symbol.valueDeclaration;
                                if (declaration && ts.isVariableDeclaration(declaration) && !declaration.type && declaration.initializer && isConstantReference(reference)) {
                                    inlineLevel++;
                                    var result = narrowType(type, declaration.initializer, assumeTrue);
                                    inlineLevel--;
                                    return result;
                                }
                            }
                        }
                    // falls through
                    case 108 /* SyntaxKind.ThisKeyword */:
                    case 106 /* SyntaxKind.SuperKeyword */:
                    case 206 /* SyntaxKind.PropertyAccessExpression */:
                    case 207 /* SyntaxKind.ElementAccessExpression */:
                        return narrowTypeByTruthiness(type, expr, assumeTrue);
                    case 208 /* SyntaxKind.CallExpression */:
                        return narrowTypeByCallExpression(type, expr, assumeTrue);
                    case 212 /* SyntaxKind.ParenthesizedExpression */:
                    case 230 /* SyntaxKind.NonNullExpression */:
                        return narrowType(type, expr.expression, assumeTrue);
                    case 221 /* SyntaxKind.BinaryExpression */:
                        return narrowTypeByBinaryExpression(type, expr, assumeTrue);
                    case 219 /* SyntaxKind.PrefixUnaryExpression */:
                        if (expr.operator === 53 /* SyntaxKind.ExclamationToken */) {
                            return narrowType(type, expr.operand, !assumeTrue);
                        }
                        break;
                }
                return type;
            }
            function narrowTypeByOptionality(type, expr, assumePresent) {
                if (isMatchingReference(reference, expr)) {
                    return getTypeWithFacts(type, assumePresent ? 2097152 /* TypeFacts.NEUndefinedOrNull */ : 262144 /* TypeFacts.EQUndefinedOrNull */);