required-properties

Validates required properties in a given object with full TypeScript support 🚀

Installation

npm i required-properties

Motivation

When we know that some property in object is not null or undefined, we can check for it. But TypeScript is not able to infer the type of original object. 😱

Usecase #1

interface Data {
    value: string | null;
}

declare function mockDbQuery(): Data

const getNonNullableData = () => {
    const dbResult = mockDbQuery()

    // by some business logic, we know that value is not null
    if (dbResult.value === null) {
        throw new Error("value is null!")
    }

    return dbResult
}

const data = getNonNullableData()
data.value // -> string | null ❌ (expected only string)

TS playground: link

Usecase #2

interface Person {
    firstName: string | null;
    lastName: string;
}

const person: Person = {
    firstName: "John",
    lastName: "Doe",
};

function printFullName(names: { firstName: string; lastName: string }) {
    console.log(`${names.firstName} ${names.lastName}`);
}

// original approach
if (person.firstName === null) {
    throw new Error("firstName is required");
}

printFullName(person); // ❌ this results in TS error, because person is still of type Person which has nullable property firstName

TS playground: link

Usage

assertRequiredProperties

• first parameter is object you want to validate
• second argument is an array of properties which can be null or undefined. You can access nested properties using dot separator

Solution for Usecase #1

import {assertRequiredProperties} from "required-properties";

interface Data {
    value: string | null;
}

declare function mockDbQuery(): Data

const getNonNullableData = () => {
    const dbResult = mockDbQuery()

    assertRequiredProperties(dbResult, ["value"]) // 👈 this changes type of dbResult

    return dbResult
}

const data = getNonNullableData()
data.value // -> string ✅

Solution for Usecase #2

import {assertRequiredProperties} from "required-properties";

interface Person {
    firstName: string | null;
    lastName: string;
}

const person: Person = {
    firstName: "John",
    lastName: "Doe",
};

function printFullName(names: { firstName: string; lastName: string }) {
    console.log(`${names.firstName} ${names.lastName}`);
}

// assertRequiredProperties approach
assertRequiredProperties(person, ["firstName"]); // 👈 this changes type of person
printFullName(person); // ✅ this is now valid TS code

It comes with full TypeScript support (also for nested nullable properties):