Informing Typescript that a variable has already been null-checked

Question

Informing Typescript that a variable has already been null-checked

An unusual yet structurally sound code snippet is presented below:

function doFoo(food: string | null = null) {
  food = food ?? getFood();  // getFood: () => string | null
  if (!food) {
    throw Error("There's still no food :(");
  }
  plate[food] = true;
}

Encountering the TypeScript error

Type 'null' cannot be used as an index type.ts(2538)

while trying to access plate[food], raises the question:

Is there a way to instruct TypeScript that the variable food will never be null when reaching that line?

typescript

Answer 1

Answer №1

Your code is expected to function correctly. Take a look at this comprehensive demonstration that showcases flawlessness:

function grabIngredient(): string | null;
const dish: { [key: string]: boolean };

function prepareMeal(ingredient: string | null = null) {
  ingredient = ingredient ?? grabIngredient();  
  if (!ingredient) {
    throw Error("Oops! Still no ingredient available :(");
  }
  dish[ingredient] = true; 
}

https://i.sstatic.net/74ZZQ.png

Answer 2

Your code is expected to function correctly. Take a look at this comprehensive demonstration that showcases flawlessness:

function grabIngredient(): string | null;
const dish: { [key: string]: boolean };

function prepareMeal(ingredient: string | null = null) {
  ingredient = ingredient ?? grabIngredient();  
  if (!ingredient) {
    throw Error("Oops! Still no ingredient available :(");
  }
  dish[ingredient] = true; 
}

https://i.sstatic.net/74ZZQ.png

Informing Typescript that a variable has already been null-checked

Answer №1

Similar questions

Managing animations with multiple components in Angular 2+

My React JS page suddenly turned blank right after I implemented a setState() function within my functional component

What is preventing type guarding in this particular instance for TypeScript?

How to implement angular 2 ngIf with observables?

Having trouble importing Bootstrap CSS into a TypeScript file

Encountered an issue with retrieving schema during self-referencing validation with openapi generator

Properly incorporating a git+https dependency

Using Typescript for testing React components: successfully passing an array of objects as props

Is it possible to customize error messages in @hapi/joi?

Certain sections within a Formik form are failing to update as intended

Issue TS1259: The module "".../node_modules/@types/bn.js/index"" can only be imported as the default using the 'esModuleInterop' flag

Enhance the annotation of JS types for arguments with default values

Retrieve the values of private variables within a defined function

Ways to link information from one entity to another

How can a particular route parameter in Vue3 with Typescript be used to retrieve an array of strings?

'This' loses its value within a function once the decorator is applied

The 'ReactNode' binding element is automatically assigned an 'any' type

The NextJS image URL remains constant across various endpoints

Having trouble mocking useAppSelector in Jest, RTL, Redux Toolkit, and React testing

Join the Observable and formControl in Angular 4 by subscribing