Uncover hidden mysteries within the object

Question

Uncover hidden mysteries within the object

I have a function that takes user input, but the argument type it receives is unknown. I need to make sure that...

value is an object
value contains a key named "a"

function x(value: unknown){
    if(value === null || typeof value !== 'object'){
        throw new Error('Expected an object');
    }

    if(!('a' in value)){
        throw new Error('Expected an object to contain property "a"');
    }
}

Typescript is giving me an error saying "Object is possibly 'null'"...

https://i.sstatic.net/jt1zT.png

How can I specifically define unknown as an object?

typescript

Answer 1

Answer №1

Interestingly, all you have to do is reverse the sequence of your validation: (sandbox)

function x(value: unknown){
    if(typeof value !== 'object' || value === null){
        // ^^^^ checking type first and then null ^^
        throw new Error('Expected an object');
    }

    if(!('a' in value)){
        throw new Error('Expected an object to contain property "a"');
    }
}

The reason for this is that confirming unknown is not null doesn't significantly narrow down the types, but by first checking typeof value !== 'object', it restricts the eligible types to object or null, making it feasible to narrow down the null scenario against that type.

Answer 2

Interestingly, all you have to do is reverse the sequence of your validation: (sandbox)

function x(value: unknown){
    if(typeof value !== 'object' || value === null){
        // ^^^^ checking type first and then null ^^
        throw new Error('Expected an object');
    }

    if(!('a' in value)){
        throw new Error('Expected an object to contain property "a"');
    }
}

The reason for this is that confirming unknown is not null doesn't significantly narrow down the types, but by first checking typeof value !== 'object', it restricts the eligible types to object or null, making it feasible to narrow down the null scenario against that type.

Uncover hidden mysteries within the object

Answer №1

Similar questions

Can I include `ChangeDetectorRef` in an Angular 4 service constructor?

Steps for confirming whether each element in the array includes the specified search string using Typescript and protractor

Tips for properly waiting for an AngularFire2 subscription to complete before executing the subsequent lines of code within Angular2

Promise<IDropdownOption[]> converted to <IDropdownOption[]>

Should data objects be loaded from backend API all at once or individually for each object?

Having trouble with updating a Firebase database object using snap.val()

What is the best way to integrate functions using an interface along with types?

Decorator in React that automatically sets the display name to match the class name

Encountered an issue: The type 'Usersinterface' is not meeting the document constraints

Angular: Comparing the Performance of Switch Statements and Dictionary Lookups

Jest encounters an issue while attempting to import Primeng CSS files

At what juncture is the TypeScript compiler commonly used to generate JavaScript code?

TypeScript encounters a self-referencing type alias circularly

Troubleshooting Issue with Chrome/chromium/Selenium Integration

Keep an ear out for updates on object changes in Angular

Challenges encountered while setting up Hotjar integration in Next.js using Typescript

How can you craft a single type that encompasses both the function signature and an array with the same structure?

What is the best way to access the data being sent to a router link in Angular?

The utilization of `ngSwitch` in Angular for managing and displaying

Cease the animated loading icon once the template has finished loading in sync with the async pipe