How to access an object's property by key in Typescript [duplicate]

Question

How do I access a property on an object type with it's key name in Typescript?

type MyType = {
  text: string;
}

var item: MyType = {
  text: "Hello World!"
}

// How do I do this?
let keyName = "text";
let key = keyName;
item[key] = "New Value";

I get this error.

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'MyType'. No index signature with a parameter of type 'string' was found on type 'MyType'.

If you declare it with const you will find that it works. That's because the type is now the literal "text" and you can access the property text. For arbitrary strings, you either have to check and cast, or use an index signature, but the former is probably preferable for this case. — k.tten
– k.tten, Commented Mar 17, 2022 at 18:03

JJWesterkamp · Accepted Answer · 2022-03-17 18:03:31Z

2

The type of key is generalised as string if you don't explicitly specify a type for it:

let key = "text" // let key: string

You can constrain values for key to only keys of MyType, after which it is okay to use it as access key for item:

let key: keyof MyType = "text" // let key: string

See a working example.

answered Mar 17, 2022 at 18:03

JJWesterkamp

7,9561 gold badge25 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Berry Blue Over a year ago

Thank you! I made one small edit to my example. I have a function returning the string for the key name. I'm still not sure how to resolve that as I still get an error if I try do this. let keyName = "text"; let key: keyof MyType = keyName;

Collectives™ on Stack Overflow

How to access an object's property by key in Typescript [duplicate]

1 Answer 1

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Linked

Related