[Typescript] Making TypeScript Stick - 5 - Extends

Let’s study a few examples of extends scenarios and see if we can figure out whether it will evaluate to true or false

64 extends number

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Answer:

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 true

 

number extends 64

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Answer:

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 false

 

 

string[] extends any

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Answer:

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 true

// does any contians string[] --- yes it is

 

 

string[] extends any[]

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Answer:

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 true

// does any[] contains string[] --- yes it is

 

 

never extends any

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Answer:

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 true

// yes, any can contain never type

 

 

 

any extends any

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Answer:

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 true

 

 

Date extends {new (...args: any[]): any }

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Answer:

false

// Date is instance type
// does not extends constructor() 

 

(typeof Date) extends {new (...args: any[]): any }

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.Answer:

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 true

// typeof Date is a constructor type