Why is the 'as' Operator in TypeScript Often Dangerous?
Available in :
Sébastien TIMONER
Expert in web development and team management, I specialize in creating and optimizing high-performance digital solutions. With extensive expertise in modern technologies like React.js, Node.js, TypeScript, Symfony, and Zephyr OS for IoT, I ensure the success of complex SaaS and IoT projects, from design to production, for companies across various sectors, at offroadLabs.
At offroadLabs, I offer custom development services that combine technical expertise with a collaborative approach. Whether creating an innovative SaaS solution, developing IoT systems with Zephyr OS, modernizing an existing application, or supporting the upskilling of a team, I am committed to delivering robust and high-performance solutions tailored to the specific needs of each project.
I am available for projects in the Aix-en-Provence area or fully remote.
You use TypeScript for the safety it brings to your code, but you've probably noticed that the as operator allows forcing compilation even when TypeScript suspects incompatible types. This flexibility can be tempting, but it often hides pitfalls. Let's discover together why excessive use of as can introduce runtime errors and how to avoid these traps.
as: Hidden Compilation ErrorsIn TypeScript, the as operator allows what's called a type assertion, meaning telling TypeScript "Trust me, I know what I'm doing." However, this tool can sometimes be a bad idea because it bypasses type checks. When you use as, you take responsibility for ensuring the type is correct, even if TypeScript can't verify it. Let's see this with some concrete examples.
as That Isn't Really CompatibleLet's take an example where you have an object of type Person with certain well-defined properties, but you decide to use as to force it to a different type, thinking it will work.
typescript
In this example, employee doesn't have the role property, but TypeScript doesn't return an error during compilation thanks to as. However, at runtime, employee.role is undefined, which can cause errors if your code expects a value of type string.
as unknown asSometimes, developers use chain casting to force a type via unknown, as in this example:
typescript
Here, we take a data value that could be anything and cast it to number via unknown. At compilation, everything seems to work, but during runtime, the addition produces NaN because data was actually a string.
Let's take another classic case where we force a partially completed object to a complete type thinking everything will be fine:
typescript
The partialProduct object is converted to Product even though it's missing the price property. This absence isn't detected by TypeScript thanks to as, but leads to undefined which can generate runtime errors if this value is used without prior checking.
To avoid problems generated by as, a good practice is to validate data at runtime, particularly if it comes from an external source. This is where Zod comes in. Zod is a TypeScript schema validation library that allows you to define secure types and validate them at runtime.
With Zod, instead of forcing a type with as, you can validate and convert data with a predefined schema. For example:
typescript
In this example, Zod checks that partialProduct respects the Product schema. If properties are missing, Zod returns a validation error instead of letting undefined values pass through. Thanks to Zod, you secure your data and avoid runtime errors related to incomplete or incorrect types.
The as operator can be a quick solution to bypass TypeScript checks, but it introduces risks of runtime errors, especially when forcing types without validation. By using a library like Zod, you can validate your data at runtime and thus fully benefit from TypeScript's safety, even in cases where as seemed like the only solution.