Question:

Type inference with 'as const' IN TypeScript

Error:

Type '{ type: string; }' is not assignable to type 'ImageFormat'.

Types of property 'type' are incompatible.

Type 'string' is not assignable to type '"image/avif" | "image/jpeg" | "image/png" | "image/svg+xml" | "image/webp"'.ts(2322)

type.ts(5, 30): The expected type comes from property 'format' which is declared here on type '{ format: ImageFormat; }'

The above error might occur because const format = { type: 'image/png' } is inferred as { type: string; } per default and type: string is not specific enough to be assignable to the type Union in ImageFormat.

A const-assertion helps here because it forces TypeScript to infer the narrowest possible type which will be the literal string 'image/png'. That way { format } is assignable to the functions return type.

Arguably it's better to just assert to ImageFormat since the literal type information you get with as const will be lost anyway after returning from the function:

type ImageFormat = {

type: 'image/avif' | 'image/jpeg' | 'image/png' | 'image/svg+xml' | 'image/webp';

};

const sampleFunction = (): { format: ImageFormat } => {

const typedFormat: ImageFormat = {

type: 'image/png',

};

const format = {

...typedFormat,

type: 'image/png' as ImageFormat["type"]

};

return {