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TypeScript Behaviors

A reference for every question, with code and detailed explanations.

1

What is the type of `Result`?

TypeScript
'Definitely not'

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2

What will TypeScript do if you write the code below?

TypeScript
TypeScript won't raise any errors

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3

What is the output of `UserWithoutBirthdate`? (assume that 'exactOptionalPropertyTypes' option in tsconfig is set to true)

TypeScript
{ name: string; age: number; }

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4

What's the type of `Result`?

TypeScript
number | string

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5

What does TypeScript infer as the return type of this function?

TypeScript
number

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6

What is the type of `Result` in the code below involving `never` type?

TypeScript
[never, true]

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7

What will TypeScript do in the following scenario with a faulty type guard?

TypeScript
TypeScript will compile without errors and the inferred type of user inside the if statement will be `string`

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8

What will TypeScript do in the following scenario with a faulty assertion function?

TypeScript
TypeScript will raise an error because after the function call `user` type is `never`, so it's not possible to use `toLowerCase()`

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9

What will TypeScript infer for the type of `UserWithoutName`?

TypeScript
{ name: string; age: number; }

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10

What is the type of `NewUser`?

TypeScript
{ name: string; age: never; }

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11

What is the type of `NameType`?

TypeScript
string

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12

What is the type of `Result`?

TypeScript
{ value: string | number; }

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13

What is the type of `Keys`?

TypeScript
"name" | "age" | "id"

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14

What is the type of `arr`?

TypeScript
(number | string | boolean)[]

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15

What is the type of `RedType`?

TypeScript
number[]

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16

What is the type of `TupleType`?

TypeScript
readonly [1, 2, 3]

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17

What will TypeScript do with the code below?

TypeScript
TypeScript won't raise any errors

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18

What is the type of `Handler`?

TypeScript
"onClick" | "onFocus"

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19

What is the type of `Result`?

TypeScript
string[]

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20

What is the type of `Result`?

TypeScript
number

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