How to specify a type based on the object returned by a function

up vote
1
down vote

favorite

If I create an object using a constructor function

function Person(name: string) this.name = name;

how would I specify the type returned by the constructor function?

function hello(person: ???): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

asked Nov 11 at 14:59

doberkofler

2,04743157

name: string ? See the docs.
– jonrsharpe
Nov 11 at 15:02

The new keyword will always return the new Object instance. It won't return the string like you are expecting.
– Ross Allen
Nov 11 at 15:04

@jonrsharpe This would imply that all object with the same properties are the same. I would imagine, that two objects with different constructor function must also be different for flow.
– doberkofler
Nov 11 at 15:06

No, it implies that all objects with the same properties have the same properties. The code consuming it just cares whether or not it has a .name it can add to 'hello '.
– jonrsharpe
Nov 11 at 15:08

1

Good question. As a workaround you can convert Person to class, and then easily use Person as a type.
– Alex
Nov 12 at 9:10

|
show 4 more comments

up vote
1
down vote

favorite

If I create an object using a constructor function

function Person(name: string) this.name = name;

how would I specify the type returned by the constructor function?

function hello(person: ???): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

asked Nov 11 at 14:59

doberkofler

2,04743157

name: string ? See the docs.
– jonrsharpe
Nov 11 at 15:02

The new keyword will always return the new Object instance. It won't return the string like you are expecting.
– Ross Allen
Nov 11 at 15:04

@jonrsharpe This would imply that all object with the same properties are the same. I would imagine, that two objects with different constructor function must also be different for flow.
– doberkofler
Nov 11 at 15:06

No, it implies that all objects with the same properties have the same properties. The code consuming it just cares whether or not it has a .name it can add to 'hello '.
– jonrsharpe
Nov 11 at 15:08

1

Good question. As a workaround you can convert Person to class, and then easily use Person as a type.
– Alex
Nov 12 at 9:10

|
show 4 more comments

up vote
1
down vote

favorite

If I create an object using a constructor function

function Person(name: string) this.name = name;

how would I specify the type returned by the constructor function?

function hello(person: ???): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

asked Nov 11 at 14:59

doberkofler

2,04743157

If I create an object using a constructor function

function Person(name: string) this.name = name;

how would I specify the type returned by the constructor function?

function hello(person: ???): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

flowtype

asked Nov 11 at 14:59

doberkofler

2,04743157

asked Nov 11 at 14:59

doberkofler

2,04743157

asked Nov 11 at 14:59

doberkofler

2,04743157

asked Nov 11 at 14:59

doberkofler

2,04743157

asked Nov 11 at 14:59

doberkofler

2,04743157

name: string ? See the docs.
– jonrsharpe
Nov 11 at 15:02

The new keyword will always return the new Object instance. It won't return the string like you are expecting.
– Ross Allen
Nov 11 at 15:04

@jonrsharpe This would imply that all object with the same properties are the same. I would imagine, that two objects with different constructor function must also be different for flow.
– doberkofler
Nov 11 at 15:06

No, it implies that all objects with the same properties have the same properties. The code consuming it just cares whether or not it has a .name it can add to 'hello '.
– jonrsharpe
Nov 11 at 15:08

1

Good question. As a workaround you can convert Person to class, and then easily use Person as a type.
– Alex
Nov 12 at 9:10

|
show 4 more comments

name: string ? See the docs.
– jonrsharpe
Nov 11 at 15:02

The new keyword will always return the new Object instance. It won't return the string like you are expecting.
– Ross Allen
Nov 11 at 15:04

@jonrsharpe This would imply that all object with the same properties are the same. I would imagine, that two objects with different constructor function must also be different for flow.
– doberkofler
Nov 11 at 15:06

No, it implies that all objects with the same properties have the same properties. The code consuming it just cares whether or not it has a .name it can add to 'hello '.
– jonrsharpe
Nov 11 at 15:08

1

Good question. As a workaround you can convert Person to class, and then easily use Person as a type.
– Alex
Nov 12 at 9:10

name: string ? See the docs.
– jonrsharpe
Nov 11 at 15:02

The new keyword will always return the new Object instance. It won't return the string like you are expecting.
– Ross Allen
Nov 11 at 15:04

@jonrsharpe This would imply that all object with the same properties are the same. I would imagine, that two objects with different constructor function must also be different for flow.
– doberkofler
Nov 11 at 15:06

No, it implies that all objects with the same properties have the same properties. The code consuming it just cares whether or not it has a .name it can add to 'hello '.
– jonrsharpe
Nov 11 at 15:08

Good question. As a workaround you can convert Person to class, and then easily use Person as a type.
– Alex
Nov 12 at 9:10

|
show 4 more comments

1 Answer
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up vote
0
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You just have to describe your constructor function in an interface as you would do with a normal class implementation and use it within your hello function:

/* @flow */

interface IPerson 
 name: string;


function Person(name: string) 
 this.name = name;


function hello(person: IPerson): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

However I would recommend against using constructor functions instead of regular classes because the point of interfaces in Flow are to be implemented by classes in order to type-guard them:

class Person implements IPerson

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

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1 Answer
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active

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1 Answer
1

active

oldest

votes

up vote
0
down vote

You just have to describe your constructor function in an interface as you would do with a normal class implementation and use it within your hello function:

/* @flow */

interface IPerson 
 name: string;


function Person(name: string) 
 this.name = name;


function hello(person: IPerson): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

However I would recommend against using constructor functions instead of regular classes because the point of interfaces in Flow are to be implemented by classes in order to type-guard them:

class Person implements IPerson

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

add a comment |

up vote
0
down vote

You just have to describe your constructor function in an interface as you would do with a normal class implementation and use it within your hello function:

/* @flow */

interface IPerson 
 name: string;


function Person(name: string) 
 this.name = name;


function hello(person: IPerson): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

However I would recommend against using constructor functions instead of regular classes because the point of interfaces in Flow are to be implemented by classes in order to type-guard them:

class Person implements IPerson

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

add a comment |

up vote
0
down vote

You just have to describe your constructor function in an interface as you would do with a normal class implementation and use it within your hello function:

/* @flow */

interface IPerson 
 name: string;


function Person(name: string) 
 this.name = name;


function hello(person: IPerson): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

However I would recommend against using constructor functions instead of regular classes because the point of interfaces in Flow are to be implemented by classes in order to type-guard them:

class Person implements IPerson

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

You just have to describe your constructor function in an interface as you would do with a normal class implementation and use it within your hello function:

/* @flow */

interface IPerson 
 name: string;


function Person(name: string) 
 this.name = name;


function hello(person: IPerson): string 
 return 'hello ' + person.name;


const person = new Person('John');
hello(person);

However I would recommend against using constructor functions instead of regular classes because the point of interfaces in Flow are to be implemented by classes in order to type-guard them:

class Person implements IPerson

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

answered Nov 19 at 3:26

Ivan Gabriele

3,28622444

add a comment |

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