Lesson 24 of 55
15 mins readPython Assertion Signatures
In Plain English
Assertion functions throw an error if a given condition is false. The `asserts value is Type` signature informs TypeScript that the variable is guaranteed to be that type for the remainder of the scope.
Core Rules to Remember
asserts condition: Eliminates null/undefined or narrows types across subsequent scope after execution.
Fail Fast: Throws descriptive errors immediately if preconditions fail.
Live Interactive Example
Hit Run Code to see it liveAsserting Type Invariants
Python 3.12
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Output Console
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How it works: After assertIsDefined executes, TypeScript eliminates null and undefined from rawPayload for the rest of the scope.
Your Turn: Micro Challenge
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Micro Exercise
Assert Number Value
- Write function assertIsNumber(val: unknown): asserts val is number that throws new Error("Not a number") if typeof val !== "number".
- Test by asserting a variable num: unknown = 42, then print (num * 2).
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Sandbox Output
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