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EasyJavaScriptTypeScript

Narrowing — typeof, instanceof & in

TypeScriptNarrowingType Guards

TypeScript uses control flow analysis to narrow types within branches. Implement describe(value) that returns a string describing its input:

InputOutput
string"string: {value}"
number (non-NaN)"number: {value}"
Date instance"date: {value.toISOString()}"
object with name property"named: {value.name}"
anything else"unknown"

Order matters — check from most specific to least specific.

Examples

  • describe("hello") → "string: hello"
  • describe(42) → "number: 42"
  • describe(new Date("2024-01-01")) → "date: 2024-01-01T00:00:00.000Z"
  • describe({ name: "Alice" }) → "named: Alice"

Sample tests

Test #1string value
Input: ["raw","hello"]
Output: "string: hello"
Test #2number value
Input: ["raw",42]
Output: "number: 42"
Test #3Date instance
Input: ["date","2024-01-01T00:00:00.000Z"]
Output: "date: 2024-01-01T00:00:00.000Z"
Test #4object with name
Input: ["raw",{"name":"Alice"}]
Output: "named: Alice"
Test #5null is unknown
Input: ["raw",null]
Output: "unknown"
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