Reid Evans posted a nice little lenses interface and composer for TypeScript. It’s missing a convenience function: a field lens for plain objects. Here is a minimal implementation:

export function field<TRec extends object, K extends keyof TRec = keyof TRec>(fieldName: K): Lens<TRec, TRec[K]> {
    return {
        get: (rec) => rec[fieldName],
        set: (rec, val) => {
            const update:Partial<TRec> = {};
            update[fieldName] = val;
            return Object.assign({}, rec, update);
        }
    };
}

interface Foo {
    bar: number;
    baz: string;
}

const fooBar = field<Foo>("bar");