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Allow switch statement type guards (need help..) #2230

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Allow switch statement type guards (need help..) #2230

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7 changes: 7 additions & 0 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -4853,6 +4853,13 @@ module ts {
narrowedType = narrowType(type, (<ConditionalExpression>node).condition, /*assumeTrue*/ child === (<ConditionalExpression>node).whenTrue);
}
break;
case SyntaxKind.CaseClause:
// In a case clause of a switch statement using typeof
var parent = (<CaseClause>node).parent;
if((<SwitchStatement>parent).expression.kind === SyntaxKind.TypeOfExpression){
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Space after if.

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Make sure the parent is guaranteed to be a switch statement. It is possible that the parser lets through case labels that are not parented by a switch. Please add either an assert or a bailout condition.

narrowedType = checkExpression((<TypeOfExpression>(<CaseClause>node).expression).text);
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This is not quite correct. You essentially need to take narrowTypeByEquality, and factor out the part that that deals with typeof, so you can access that part here. You need to check that the node.expression is a StringLiteral, and then cast it to a LiteralExpression to get the text. The TypeOfExpression in this case would be parent.expression.

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As you'll see in your build results, you got

src/compiler/checker.ts(4860,114): error TS2339: Property 'text' does not exist on type 'TypeOfExpression'.

}
break;
case SyntaxKind.BinaryExpression:
// In the right operand of an && or ||, narrow based on left operand
if (child === (<BinaryExpression>node).right) {
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