plainToClass assumes response is an array

You are having this issue because Typescript does not know the type of response.json(). This is because plainToClass (and some other methods from class-transformer) has multiple signatures. From src/index.ts:

/**
 * Converts plain (literal) object to class (constructor) object. Also works with arrays.
 */
export function plainToClass<T, V extends Array<any>>(cls: ClassType<T>, plain: V, options?: ClassTransformOptions): T[];
export function plainToClass<T, V>(cls: ClassType<T>, plain: V, options?: ClassTransformOptions): T;
export function plainToClass<T, V>(cls: ClassType<T>, plain: V|V[], options?: ClassTransformOptions): T|T[] {
    return classTransformer.plainToClass(cls, plain as any, options);
}

Since Typescript cannot determine the appropriate signature, I assume it just picks the first one, hence why it’s telling you that Proposal[] is expected.

To fix your problem, you simply need to explicitly declare the type of response.json() using as Proposal:

.then(response => plainToClass(Proposal, response.json() as Proposal));

And off you go 😃

To array:

plainToClass(Permission, <any[]>JSON.parse(response))

To single:

plainToClass(Permission, <any>JSON.parse(response))

^-- a little less redundant.