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The Hidden Dangers of Assuming TypeScript Covers All Your Bases

The Illusion of Safety

As developers, we often assume that using TypeScript ensures our code is error-free and secure. But, what if I told you that’s not entirely true? In this section, we’ll explore the limitations of TypeScript and how it can still lead to errors, even with proper type checking.

interface ExampleType {
  name: string;
  pets: { name: string; legs: number }[];
  age?: number;
}

const fetchData = () => {
  // fetch data from an external API
};

const getBiped = (undefined ExampleType) => {
  return data.pets.find((pet) => pet.legs === 2);
};

At first glance, this code looks fine. We’ve defined a type ExampleType with required properties name and pets, and an optional property age. We’ve also created a function getBiped that takes an ExampleType object and returns a pet with two legs.

But, what happens when we execute this code? You might be surprised to find that it throws an error: Cannot read property ‘find’ of undefined. Why? Because the external API returned an object without the pets property, and our code didn’t account for that possibility.

The Importance of Dynamic Validations

TypeScript’s static type checking is excellent, but it’s not a silver bullet. We need to complement it with dynamic validations to ensure our code is robust and error-free. The question is, how do we validate our objects effectively?

Manual Validation

One approach is manual validation, where we write custom validation functions to check the structure of our objects. For example:

const validate = (undefined ExampleType) => {
  if (!data.pets) {
    throw new Error('pets property is required');
  }
  // additional checks...
};

While this approach works, it has its drawbacks. We need to write and maintain custom validation code, which can be time-consuming and prone to errors.

Using a Validation Library

Another approach is to use a validation library, such as ajv or joi. These libraries provide a standardized way to define schemas and validate objects against them. For example:

import { ajv } from 'ajv';

const schema = {
  type: 'object',
  properties: {
    name: { type: 'string' },
    pets: {
      type: 'array',
      items: {
        type: 'object',
        properties: {
          name: { type: 'string' },
          legs: { type: 'number' },
        },
      },
    },
  },
};

const validate = ajv.compile(schema);

const data = { name: 'John', pets: undefined };
if (!validate(data)) {
  console.error(validate.errors);
}

Validation libraries offer many benefits, including standardized validation and improved error reporting. However, they require additional setup and maintenance.

Dynamic Type Validators

The third approach is to use dynamic type validators, which generate validation functions from our type definitions. This approach combines the best of both worlds: the safety of static type checking and the flexibility of dynamic validation.

For example, using the typescript-json-validator library, we can generate a validation function from our ExampleType definition:

import { generateValidator } from 'typescript-json-validator';

const validator = generateValidator(ExampleType);

const data = { name: 'John', pets: undefined };
if (!validator(data)) {
  console.error(validator.errors);
}

This approach is powerful and flexible, allowing us to generate validation functions from our type definitions.

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