Deciding the right type guard
We have seen 3 different methods to implement type guards:
- Type predicates
- Equality Checks
- Assertions
Here's a brief summary of when to prefer type predicates, equality checks, or assertions in TypeScript:
1. Type Predicates (value is Type
)
- Use When: You need to determine the type of a value within a conditional block and ensure TypeScript narrows the type based on the check.
- Best For: Situations where type checks are common, reusable, and where you want the function to return
true
orfalse
.
Example: Checking if an event is of a specific type in a function that processes different types of events.
ts
function isClickEvent(event: Event): event is ClickEvent {return event.type === "click";}
2. Equality Checks (===
)
- Use When: You need a straightforward comparison of values.
- Best For: Simple cases where you're comparing primitives or checking specific conditions. Easier to implement when the check is simple and not re-used many times, hence adding a type predicate may not be beneficial.
Example: Verifying that a string or number has a particular value.
ts
if (event.type === "click") {console.log("This is a ClickEvent.");}
3. Assertion Functions (asserts value is Type
)
- Use When: You want to enforce that a value must be of a specific type and want to throw an error if it isn't.
This stops further execution if the assertion fails.
- Best For: Scenarios where it's critical that a value conforms to a specific type, and you need to ensure that downstream code operates on the correct type without fallback logic.
Example:
ts
function assertIsClickEvent(event: Event): asserts event is ClickEvent {if (event.type !== "click") {throw new Error("Not a ClickEvent");}}
Summary:
- Use Type Predicates for reusable type checks where the result determines code flow.
- Use Equality Checks for simple, direct comparisons or when TypeScript can infer the correct type from the comparison.
- Use Assertion Functions when you need to guarantee a specific type and want to stop execution if the type condition isn't met.