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Excess properties in TypeScript

· 2 min read
Filip Tammergård
Software Engineer at Einride

Properties defined on an object that are not expressed by the type of the object are called excess properties.

Here's an example:

type Person = {
firstName: string

const person: Person = {
firstName: "Filip",
lastName: "Tammergård", // ⛔️ Object literal may only specify known properties, and 'lastName' does not exist in type 'Person'.

In this case, TypeScript makes it clear that lastName is not defined in the Person type and is therefore not allowed.