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Why we need @types in TypeScript


By : Jose Moreira
Date : September 17 2020, 02:00 AM
seems to work fine You do not have to install @types/lodash package, however, it is good to install it. The package contains type definitions, so your IDE can smartly hint you and you will be able to strictly define types for parameters etc.
To be accurate, @types/lodash is not a TypeScript version of Lodash - it is only package with type definitions, so you have to install Loadash via npm i --save lodash to be able to use it and optionally download the type package.
code :


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Typescript syntax to loop through types -- let type Types = typeof myObject[number]


By : adsffdsa
Date : March 29 2020, 07:55 AM
Hope that helps I was also intrigued by this syntax . Turns out this is what you call a lookup type. It's best explained in the annoucement blog of Typescript 2.1 here.
In your case things are much easier to understand if we explicitly write out the type,
code :
interface Actions {
  [index: number] = ActionType1 | ActionType2
}
const returnsOfActions: Actions = Object.values(actions).map($call);
export type TodosAction = typeof returnsOfActions[number];

How do I add TypeScript types to a JavaScript module without switching to TypeScript?


By : user2172969
Date : March 29 2020, 07:55 AM
wish of those help What you will want to create are Definition Files. These files are used to describe your JavaScript.
Let's say for simplicity you have a function called add, it will take two numbers or an array of numbers.
code :
exports.add = function add(a, b) {
  if(args.length == 1) return a.reduce((sum, val) => sum + val, 0)
  return a + b; 
}
/**
 * Sums up two numbers.
 *
 * @param {number} a The first number to add.
 * @param {number} b The second number to add.
 * @returns {number} The resulting sum of the two numbers.
 */
export declare function add(a: number, b: number): number
/**
 * Sums up a list of numbers.
 *
 * @param {number[]} numbers An array of numbers to add.
 * @returns {number} The resulting sum of all the numbers.
 */
export declare function add(numbers: number[]): number
{
  "types": "types/index.d.ts"
}

TypeScript can't find module flatpickr, even though it includes typescript types


By : Mindaugas Steponavič
Date : March 29 2020, 07:55 AM
To fix this issue Your tsconfig.json uses "module": "es6". According to the compiler options definition, this causes the compiler to lookup modules in a classic manner. The classic lookup works for some scenarios but does not support node modules defined via packages.json, which is the case for flatpickr.
What you need to change for this to work for you is either set the tsconfig.json to "moduleResolution": "node" or perhaps use "module": "commonjs" which will switch both the emitted modules and resolution mechanism to node.js standards.

Typescript Conditional Types combined with mapped types documentation example clarification


By : user3187497
Date : March 29 2020, 07:55 AM
To fix the issue you can do It's called a lookup type
Using the array access syntax we can get a union of all property types that we pass in.
code :
({ a: string, b: number, c: object })['a' | 'b'] // -> string | number
{
  a: () => void,
  b: () => void,
  c: string
}
{
  a: 'a',
  b: 'b',
  c: never
}
({ a: 'a', b: 'b', c: never })['a' | 'b' | 'c'] // -> 'a' | 'b' | never

How to allow partial TypeScript types when mocking functions - Jest, TypeScript


By : yash
Date : March 29 2020, 07:55 AM
This might help you I have a function which I would like to mock for testing purposes in TypeScript. In my tests, all I care about are the json and the status. However, when using Jest's jest.spyOn the type of my mocked function is set to return a http Response type. This is awkward as it means I have to manually go and implement a bunch of functions and properties that are irrelevant and arbitrary. , You can use as...
code :
export function mockApi(json: object, status: number): void {
  jest.spyOn(
    myApiModule,
    'methodWhichReturnsAResponse'
  ).mockImplementation(() =>
    Promise.resolve({
      json: () => Promise.resolve(json),
      status
    } as http.Response), // <-- here
  );
}
type X {
  a: number
  b: number
}

const x = { a: 2 } as X // OK
const y = { a: 3, c: 2 } as X // NOT OK, because c does not exist in X
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