Type narrowing doens't work as expected for objectHow to tell TypeScript that optional property in object exists and is set?Type definition in object literal in TypeScriptAre strongly-typed functions as parameters possible in TypeScript?Typescript: Interfaces vs TypesTypeScript. How to use not exported type definitions?Typescript check if property in object in typesafe wayWhy doesn't typescript undefined type behave same as optional?TypeScript Inline static type guardType for non-falsy, aka truthykeyof interface without generic index signaturesHow can I check that a string is a property a particular interface in TypeScript
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Type narrowing doens't work as expected for object
How to tell TypeScript that optional property in object exists and is set?Type definition in object literal in TypeScriptAre strongly-typed functions as parameters possible in TypeScript?Typescript: Interfaces vs TypesTypeScript. How to use not exported type definitions?Typescript check if property in object in typesafe wayWhy doesn't typescript undefined type behave same as optional?TypeScript Inline static type guardType for non-falsy, aka truthykeyof interface without generic index signaturesHow can I check that a string is a property a particular interface in TypeScript
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
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
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
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
;
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
typescript
asked Mar 8 at 9:38
chatoo2412chatoo2412
9815
asked Mar 8 at 9:38
chatoo2412chatoo2412
9815
asked Mar 8 at 9:38
chatoo2412chatoo2412
9815
asked Mar 8 at 9:38
chatoo2412chatoo2412
9815
asked Mar 8 at 9:38
chatoo2412chatoo2412
9815
9815
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
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.
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
add a comment |
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1 Answer
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
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.
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
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.
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
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.
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
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.
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
answered Mar 8 at 9:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
71.6k35068
71.6k35068
add a comment |
add a comment |
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