diff.js

let ref = {
	foo: 123,
	bar: "test"
};

let obj = {
	foo: 123,
	bar: "what"
}

interface Patch {
	empty: boolean;
	value?: any;
}

function genericDiff(obj: any, ref: any): Patch {
	if (obj === ref) {
		return { empty: true };
	}
	if (Array.isArray(obj)) {
		return arrayDiff(obj, ref);
	}
	if (typeof obj === "object") {
		return objectDiff(obj, ref);
	}
	return primitiveDiff(obj, ref);
}

function primitiveDiff(obj: any, ref: any): Patch {
	return {
		empty: obj === ref,
		value: obj
	};
}

function arrayDiff(obj: Array<any>, ref: Array<any>): Patch {
	if (obj.length !== ref.length) {
		return { empty: false, value: obj };
	}
	for (let i = 0; i < obj.length; i++) {
		let p = genericDiff(obj[i], ref[i]);
		if (!p.empty) {
			return { empty: false, value: obj }
		}
	}
	return { empty: true };
}

function objectDiff(obj: any, ref: any): Patch {
	let patch = { empty: true, value: {} } as Patch;
	for (let key of Object.keys(ref)) {
		let p = genericDiff(obj[key], ref[key]);
		if (!p.empty) {
			patch.empty = false;
			patch.value[key] = p.value;
		}
	}
    return patch;
}

let patch = genericDiff({ foo: ["b"] }, { foo: ["b"] });

console.log(JSON.stringify(patch.value));