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Type narrowing doens't work as expected for object

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;

0

interface Obj 
 foo?: string;
 bar?: string;


const fn = (obj: Obj, key: keyof Obj) => 
 if (obj.foo) 
 const val1 = obj.foo; // string
 

 if (obj['foo']) 
 const val2 = obj['foo'] // string
 

 const picked = obj[key];

 if (picked) 
 const val3 = picked; // string
 

 if (obj[key]) 
 const val4 = obj[key]; // string 
;

I wonder why type narrowing doesn't work in case of val4.

I know I can use ! or custom defined type guards answered here. But what I want to know is the reason.

typescript

share|improve this question

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

add a comment | 

0

interface Obj 
 foo?: string;
 bar?: string;


const fn = (obj: Obj, key: keyof Obj) => 
 if (obj.foo) 
 const val1 = obj.foo; // string
 

 if (obj['foo']) 
 const val2 = obj['foo'] // string
 

 const picked = obj[key];

 if (picked) 
 const val3 = picked; // string
 

 if (obj[key]) 
 const val4 = obj[key]; // string 
;

I wonder why type narrowing doesn't work in case of val4.

I know I can use ! or custom defined type guards answered here. But what I want to know is the reason.

typescript

share|improve this question

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

add a comment | 

interface Obj 
 foo?: string;
 bar?: string;


const fn = (obj: Obj, key: keyof Obj) => 
 if (obj.foo) 
 const val1 = obj.foo; // string
 

 if (obj['foo']) 
 const val2 = obj['foo'] // string
 

 const picked = obj[key];

 if (picked) 
 const val3 = picked; // string
 

 if (obj[key]) 
 const val4 = obj[key]; // string 
;

I wonder why type narrowing doesn't work in case of val4.

I know I can use ! or custom defined type guards answered here. But what I want to know is the reason.

typescript

share|improve this question

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

interface Obj 
 foo?: string;
 bar?: string;


const fn = (obj: Obj, key: keyof Obj) => 
 if (obj.foo) 
 const val1 = obj.foo; // string
 

 if (obj['foo']) 
 const val2 = obj['foo'] // string
 

 const picked = obj[key];

 if (picked) 
 const val3 = picked; // string
 

 if (obj[key]) 
 const val4 = obj[key]; // string 
;

share|improve this question

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

share|improve this question

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

asked Mar 8 at 9:38

chatoo2412chatoo2412

9815

1 Answer
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Narowing does not work on dynamic access to the object (access with [] and a variable as the index). This limitation is due to performance reasons. You can search GitHub for issues around this, here is one.

share|improve this answer

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

add a comment | 

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0

Narowing does not work on dynamic access to the object (access with [] and a variable as the index). This limitation is due to performance reasons. You can search GitHub for issues around this, here is one.

share|improve this answer

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

add a comment | 

0

Narowing does not work on dynamic access to the object (access with [] and a variable as the index). This limitation is due to performance reasons. You can search GitHub for issues around this, here is one.

share|improve this answer

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

add a comment | 

Narowing does not work on dynamic access to the object (access with [] and a variable as the index). This limitation is due to performance reasons. You can search GitHub for issues around this, here is one.

share|improve this answer

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

share|improve this answer

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370

answered Mar 8 at 9:48

Titian Cernicova-DragomirTitian Cernicova-Dragomir

73.4k35370