Typscript error when accessing object's value by string index

February 12, 2024

In javascript, accessing object value via obj.field can be done by obj['field'] too. But produces error in typescript.

function F() {
  const obj = { field: "firstname", title: "First Name", value: 10 };
  let field = "field"
  console.log(obj[field]); // <-- Typescript error: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ field: string; title: string; value: number; }'
}

How can I cast obj to resolve typescript error?

>Solution :

You can narrow field as follows:

function F() {
  const obj = { field: 'firstname', title: 'First Name', value: 10 };
  const field: keyof typeof obj = 'field'; // field is narrowed to be a known key of obj instead of 'string'

  console.log(obj[field]); // no type error here
}