Questions tagged [typeguards]
Use for questions about narrowing types with predefined (like "instanceof", "typeof" and "in" operators) or user-defined guards.
typeguards
235
questions
2
votes
1
answer
40
views
TypeScript type guards. How to get type narrowing AND suggestions?
I have assertIsDefined type guard with expected value.
I need to type narrowing to expected value with type suggestions.
For type narrowing i need to add | unknown to generic type, and suggestions ...
- 23
2
votes
1
answer
48
views
Why is the behavior inconsistent when intersecting type guards?
I want to use type guards so that hasNext can filter out types.
type Queue1 = {
operation: 'move';
offset: number;
};
type Queue2 = {
operation: 'eat';
food: string;
};
type Queue3 = {
...
- 23
1
vote
1
answer
22
views
Trying to add a typeguard dependent on content existing in the element in Typescript
I'm trying to create a conditional in Typescript that checks if a specific word already exists in the name.
I have this function:
isOrganic() {
for (let i = 0; i < this.items.length; i++) {
...
3
votes
1
answer
93
views
Why write the return type of a type guard as `this is (…) & this` in TypeScript?
I'm reading the Classes section of the handbook, but there is a difference that makes me confused.
In the following examples:
In Example 1, they first explicitly wrote the return type this is ...
- 2,609
1
vote
1
answer
56
views
Type guarding class in switch statement
I haven't use Typescript in many years and can't remember nor find how to type guard many classes inside a switch statement correctly.
class A {}
class B {}
class C {}
type OneOfThem = A | B | C;
...
- 878
1
vote
1
answer
64
views
How to define TypeScript return type in a method to infer variable existence based on a parameterized type?
I have a TypeScript class called Design which contains a method named checkFetched. This method is designed to check if a property of type DesignData exists within the class instance based on a ...
- 23
1
vote
0
answers
31
views
in typescript, how to write a generic function that returns the typeof [duplicate]
Here is my TypeScript code:
interface TheType{
name:string
}
function get_ts_type<T>(): string {
return // todo: implement this
}
function test_it(){
const ans=get_ts_type<TheType>()
...
- 4,443
0
votes
1
answer
56
views
Is it possible to document with JSDoc a sort of type guard for document.getElementById?
The idea here is to generalize the following pattern:
const element = document.getElementById('foo')
if (!element || !(element instanceof HTMLInputElement)) throw new Error('Failed to locate HTML ...
- 1,361
1
vote
1
answer
44
views
Why isn't type guard applied in arguments of callback functions? [duplicate]
The following is a simple TypeScript code example.
// Example Setup
declare var foo:{bar?: {baz: string}};
function immediate(callback: () => void) {
callback();
}
// Type Guard
if (foo.bar) {
...
- 333
0
votes
1
answer
51
views
Limit argument types for guard function
How to implement a guard function isUndefined that only accept arguments whose type includes undefined? The function should assert that the argument is undefined.
Basic implementation:
function ...
- 3
-1
votes
1
answer
51
views
Assert is typeguard
Can anybody explain to me why in the console log the notUser has a type
const notUser: {
items: any[];
} & User
I expect it to be User
interface User {
id: string
}
function assVal (obj:any)...
- 670
1
vote
1
answer
167
views
Why TypeScript type guard not narrowing down the union?
In the following TypeScript code, I expect contact to be narrowed down to GroupContact due to the type guard in place, but contact is still Contact | GroupContact. Why?
type Contact = {
id: string;...
- 170k
0
votes
1
answer
25
views
How to properly find array element and cast when array has union types and generics?
I have the following type declarations:
types.ts:
type ItemKind = 'A' | 'B';
type ValidItem<TItemKind extends ItemKind = ItemKind> = {
readonly type: TItemKind;
readonly id: number;
};
...
- 3,452
1
vote
1
answer
127
views
How to create a TypeGuard that mimics isinstance
I have to check an object that may have been created by an API.
When I try using isinstance(obj, MyClass) it get a TypeError if obj was created by the API.
I wrote a custom function to handle this.
...
- 177
3
votes
0
answers
162
views
Python generic type annotation for pandas series
I am encountering a type checking error when running my code through typeguard.
I the following file in my package:
from __future__ import annotations
import pandas as pd
def col_sum(x: pd.Series[...
- 1,084