Generic constraint when defining a type
I am trying to come up with a type for a function with a single callback parameter (also a function) such that the type of the parameter of that callback is restricted to object only.
However I am getting a type error. Here is the code:
function aaa(a: { n: number }) { }
type Cb = <T extends object>(a: T) => void
function ccc(fn: Cb) { }
// type error - why?
ccc(aaa)
Type error:
Argument of type '(a: { n: number; }) => void' is not assignable to parameter of type 'Fn'.
Types of parameters 'a' and 'a' are incompatible.
Type 'T' is not assignable to type '{ n: number; }'.
Type 'object' is not assignable to type '{ n: number; }'.
Property 'n' is missing in type '{}'.
A similar generic constraint, but applied to a function definition, works fine:
function aaa(a: { n: number }) { }
function bbb<T extends object>(fn: (a: T) => void) { }
// all good
bbb(aaa)
What is the difference between the two? And how can I get the former one to work?
Thanks!
EDIT
playground link
typescript typescript-typings
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
add a comment |
I am trying to come up with a type for a function with a single callback parameter (also a function) such that the type of the parameter of that callback is restricted to object only.
However I am getting a type error. Here is the code:
function aaa(a: { n: number }) { }
type Cb = <T extends object>(a: T) => void
function ccc(fn: Cb) { }
// type error - why?
ccc(aaa)
Type error:
Argument of type '(a: { n: number; }) => void' is not assignable to parameter of type 'Fn'.
Types of parameters 'a' and 'a' are incompatible.
Type 'T' is not assignable to type '{ n: number; }'.
Type 'object' is not assignable to type '{ n: number; }'.
Property 'n' is missing in type '{}'.
A similar generic constraint, but applied to a function definition, works fine:
function aaa(a: { n: number }) { }
function bbb<T extends object>(fn: (a: T) => void) { }
// all good
bbb(aaa)
What is the difference between the two? And how can I get the former one to work?
Thanks!
EDIT
playground link
typescript typescript-typings
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46
add a comment |
I am trying to come up with a type for a function with a single callback parameter (also a function) such that the type of the parameter of that callback is restricted to object only.
However I am getting a type error. Here is the code:
function aaa(a: { n: number }) { }
type Cb = <T extends object>(a: T) => void
function ccc(fn: Cb) { }
// type error - why?
ccc(aaa)
Type error:
Argument of type '(a: { n: number; }) => void' is not assignable to parameter of type 'Fn'.
Types of parameters 'a' and 'a' are incompatible.
Type 'T' is not assignable to type '{ n: number; }'.
Type 'object' is not assignable to type '{ n: number; }'.
Property 'n' is missing in type '{}'.
A similar generic constraint, but applied to a function definition, works fine:
function aaa(a: { n: number }) { }
function bbb<T extends object>(fn: (a: T) => void) { }
// all good
bbb(aaa)
What is the difference between the two? And how can I get the former one to work?
Thanks!
EDIT
playground link
typescript typescript-typings
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
I am trying to come up with a type for a function with a single callback parameter (also a function) such that the type of the parameter of that callback is restricted to object only.
However I am getting a type error. Here is the code:
function aaa(a: { n: number }) { }
type Cb = <T extends object>(a: T) => void
function ccc(fn: Cb) { }
// type error - why?
ccc(aaa)
Type error:
Argument of type '(a: { n: number; }) => void' is not assignable to parameter of type 'Fn'.
Types of parameters 'a' and 'a' are incompatible.
Type 'T' is not assignable to type '{ n: number; }'.
Type 'object' is not assignable to type '{ n: number; }'.
Property 'n' is missing in type '{}'.
A similar generic constraint, but applied to a function definition, works fine:
function aaa(a: { n: number }) { }
function bbb<T extends object>(fn: (a: T) => void) { }
// all good
bbb(aaa)
What is the difference between the two? And how can I get the former one to work?
Thanks!
EDIT
playground link
typescript typescript-typings
typescript typescript-typings
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
edited Nov 25 '18 at 15:20
artemave
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
asked Nov 25 '18 at 15:13
artemaveartemave
3,42853963
3,42853963
related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46
add a comment |
related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46
related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46
related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46
add a comment |
1 Answer
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The problem is that the regular function aaa is not compatible with the generic function signature Cb.
You probably want to declare Cb as a normal function signature, but with Cb having the generic type parameter
function aaa(a: { n: number }) { }
type Cb<T extends object> = (a: T) => void
function ccc<T extends object>(fn: Cb<T>) { }
// ok
ccc(aaa)
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
add a comment |
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1 Answer
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1 Answer
1
active
oldest
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oldest
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oldest
votes
The problem is that the regular function aaa is not compatible with the generic function signature Cb.
You probably want to declare Cb as a normal function signature, but with Cb having the generic type parameter
function aaa(a: { n: number }) { }
type Cb<T extends object> = (a: T) => void
function ccc<T extends object>(fn: Cb<T>) { }
// ok
ccc(aaa)
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
add a comment |
The problem is that the regular function aaa is not compatible with the generic function signature Cb.
You probably want to declare Cb as a normal function signature, but with Cb having the generic type parameter
function aaa(a: { n: number }) { }
type Cb<T extends object> = (a: T) => void
function ccc<T extends object>(fn: Cb<T>) { }
// ok
ccc(aaa)
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
add a comment |
The problem is that the regular function aaa is not compatible with the generic function signature Cb.
You probably want to declare Cb as a normal function signature, but with Cb having the generic type parameter
function aaa(a: { n: number }) { }
type Cb<T extends object> = (a: T) => void
function ccc<T extends object>(fn: Cb<T>) { }
// ok
ccc(aaa)
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
The problem is that the regular function aaa is not compatible with the generic function signature Cb.
You probably want to declare Cb as a normal function signature, but with Cb having the generic type parameter
function aaa(a: { n: number }) { }
type Cb<T extends object> = (a: T) => void
function ccc<T extends object>(fn: Cb<T>) { }
// ok
ccc(aaa)
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
answered Nov 25 '18 at 15:48
Titian Cernicova-DragomirTitian Cernicova-Dragomir
70k34866
70k34866
add a comment |
add a comment |
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related: stackoverflow.com/questions/50933740/…
– artem
Nov 25 '18 at 15:46