Lesson 23 of 55
15 mins readPython User-Defined Type Guards
In Plain English
When built-in typeof or in checks are not enough, user-defined type guards allow you to create custom functions returning a type predicate `param is Type` that instructs the TypeScript compiler.
Core Rules to Remember
Type Predicates: Functions returning arg is Type act as custom compiler narrowing guards.
Runtime + Compile Safety: Runs standard JavaScript boolean logic at runtime while narrowing types at compile time.
Live Interactive Example
Hit Run Code to see it liveCreating Custom Type Predicates
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: Inside the if (isAdmin(u1)) branch, u1 is automatically narrowed to AdminUser.
Your Turn: Micro Challenge
No pressure! Edit the starter code below and test your solution with instant feedback.
Micro Exercise
Build a Non-Empty String Guard
- Write type guard function isString(val: unknown): val is string that checks typeof val === "string".
- Write function printUpper(val: unknown): void that logs val.toUpperCase() if isString(val) is true, else logs "NOT_A_STRING".
- Call printUpper("zen") and printUpper(123).
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Sandbox Output
Click "Run & Check" to test your solution.
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