Lesson 37 of 55
15 mins readPython The keyof Type Operator
In Plain English
The keyof operator takes an object type and produces a string or numeric literal union of its keys, allowing dynamic metaprogramming.
Core Rules to Remember
Literal Union of Keys: Produces a union of all property names of a type.
Metaprogramming Foundation: Forms the building block for mapped and indexed access types.
Live Interactive Example
Hit Run Code to see it liveExtracting Keys from Types
Python 3.12
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Output Console
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How it works: keyof SystemConfig generates the literal union "host" | "port" | "timeout" | "sslEnabled".
Your Turn: Micro Challenge
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Micro Exercise
Enforce Allowed Metrics Keys
- Define interface NodeMetrics { cpu: number; memory: number; disk: number }.
- Create type MetricName = keyof NodeMetrics.
- Write function trackMetric(metric: MetricName, val: number): void logging `${metric}: ${val}`.
- Call trackMetric("cpu", 85).
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Sandbox Output
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