Errata

function getRating(ratings: Ratings, key: 'audience' | 'critic'): number {
return ratings[key]; // Ok
}

const ratings: Ratings = { audience: 66, critic: 84 };

getCountLiteral(ratings, 'audience'); // Ok

getCountLiteral(ratings, 'not valid');

There is no getCountLiteral its supposed to be
getRating(ratings, 'audience'); // Ok
getRating(ratings, 'not valid');

Note from the Author or Editor:
Thanks for the catch!

Here's the excerpt from the book
// Type is either a number or an array of strings
let stringOrArrayOfNumbers: string | number[];

where the comment above should actually be
// Type is either a string or an array of numbers

Note from the Author or Editor:
You're right, thanks for the spot!

Error states
Error: Type '{ genre: string; name: string; strategy: string; }'
is not assignable to type 'Novella'.
Object literal may only specify known properties,
and 'genre' does not exist in type 'Novella'.
when it should be
Error:Type '{ pages: number; strategy: string; style: string; }'
is not assignable to type 'Novella'.
Object literal may only specify known properties,
and 'strategy' does not exist in type 'Novella'.

Note from the Author or Editor:
Nice spot, thanks!

In the type error for the `const declared: Entertainer` expression, it mentions " Error: Property 'acts' is missing in type '{ one: number; }' but required in type 'Entertainer'.". That object should be `{ name: string; }` instead.

Note from the Author or Editor:
Thanks for the spot!

In the code sample, it says that the following should throw an error becasue "Property 'author' is missing in type..."

interface Book {
author?: string;
pages: number;
}

const missing: Book = {
pages: 80
}

But, it's actually okay as the 'author' property is optional.

The following will indeed be wrong because it's missing the 'pages' property, which is required:

const missing: Book = {
author: "Rita Dove"
}

Note from the Author or Editor:
Thanks for the spot!

In the code snippet

```
interface FunctionWithCount {
count: number;
(): void;
}

let hasCallCount: FunctionWithCount;

function keepsTrackOfCalls() {
keepsTrackOfCalls.count +=1;
console.log(`I've been called ${keepsTrackOfCalls.count} times!`)
}

keepsTrackOfCalls.count = 0

hasCallCount = keepsTrackOfCalls

function doesNotHaveCount() {
console.log('No Idea!')
}

hasCallCount = doesNotHaveCount
```

Error is listed as
```
"Property 'count' is missing in type '() => void' but required in type 'FunctionWithCalls'."
```

Should be
```
"Property 'count' is missing in type '() => void' but required in type 'FunctionWithCount'."
```

Note from the Author or Editor:
Nice spot, thanks!

'@param {Painting} painter' should be '@param {Painter} painter'

Note from the Author or Editor:
Indeed, nice spot. Thank you!

In the Optional properties section [author?: string;] is optional, there is no error for [missing], in the graphic it shows the false error "// Error: Property 'author' is missing in type" for [missing]
type Book = {
author?: string;
pages: number;
};

const missing: Book = {
pages: 80
};

`'{ name: string; start: number; }'` should be `'{ born: number; name: string; }`

Change "// Error: Type 'string' is not assignable to 'Poet'" to "// Error: Type 'string' is not assignable to type 'Poet'" in order to include the word "type" and be technically accurate.

Note from the Author or Editor:
Thanks for the spot!

The error message shown in VS Code is:
"Property 'pages' does not exist on type 'Poem'."

Change "Property 'pages' does not exist on type 'PoemWithPages | PoemWithRhymes'." to "Property 'pages' does not exist on type 'Poem'."

Note from the Author or Editor:
Thanks for the spot!

Change "console.log(`${songs}`);" to "console.log(`${song}`);" so it matches the non-plural form of the parameter name.

Note from the Author or Editor:
Thanks for the spot!

... attempting to pass the entire `[string, [number, boolean]` as the first parameter ...

misses the closing bracket I think and should read

... attempting to pass the entire `[string, [number, boolean] ]` as the first parameter ...

Note from the Author or Editor:
Agreed, thanks for the spot!

In the provided code example, "author" is an optional property, so it is not "missing" as suggested in the "// Error Property 'author' is missing in type..."

Note from the Author or Editor:
Thanks for the spot!

const pageIsh = {
text:
"Hello, world!",
};
// Ok: messengerIsh is an inferred object type with text, not a Page
page.text += "!";
// Ok: read takes in Page, which happens to
// be a more specific version of pageIsh's type
read(messengerIsh);

What is "messengerIsh"? Did you mean "pageIsh" (2x)?

Please provide the correct code listing as answer if there is something wrong, thank you!

Note from the Author or Editor:
Indeed, thanks for the spot!

I test new WithMethod().myProperty === new WithMethod().myProperty;
book expects it is false. but my result is true.
chatgpt said, "This may be because the TypeScript compiler is optimizing the code and replacing the two separate functions with a single shared function. This optimization is possible because the myProperty function has no side effects and always returns void, so it is safe to reuse it."
I request it is right or not.

Note from the Author or Editor:
Thanks for the report! Indeed, the class should assign an initial value to the myProperty property:

class WithProperty {
myProperty = () => {}
}

Right now it just defines the type of the property without assigning an initial value. So new WithMethod().myProperty will be undefined.

By the way, I would strongly discourage anybody from asking ChatGPT about type checking or other intricate programming operations. It can get most basic & common questions right but is often shockingly wrong about more advanced topics. In this case, it's *very* wrong about TypeScript. TypeScript's compiler performs almost no code optimization, and certainly doesn't optimize by replacing separate functions with a single shared function. You can check how the compiler acts on typescriptlang.org/play.

ChatGPT and other generative AI stuff is very nifty but not ready yet for this kind of question. ????

Change "This OptionalProperty class..." to "This MissingInitializer class...".

Note from the Author or Editor:
Thanks for the spot!

The assignment in the Quote constructor has a syntax error. Instead of `this.text = ;`, it should be `this.text = text;`.

Note from the Author or Editor:
Thanks for the spot!

on p.109 first line, Quote.text = "Ha!" // maybe quote.text = "Ha!" ?

class Quote {
readonly text: string;

constrcutor(text: string) {
this.text = text; // changed by other reader's report
}
}

const quote = new Quote(
"There is..."
)

constructor is not working here.
error message: Cannot assign to 'text' because it is a read-only property. // Expected 0 arguments, but got 1.

Note from the Author or Editor:
Indeed - I don't know how that 'Q' became capitalized. Thanks for the report!

It should say "Error: Property 'value' in type 'VagueGrade' is not assignable to the same property in base type 'NumericGrade'."

Note from the Author or Editor:
Thanks for the spot!

Change "This HasStatic class..." to "This Question class..." in order to match the class name below it.

Note from the Author or Editor:
Thanks for the spot!

Change "HasStatic" to be "Question" in the error code example.

Note from the Author or Editor:
Thanks for the spot!

For the first if branch inside of the greetComedianSafety() function, change it from "if (typeof value === "string")" to "if (typeof name === "string")" so it matches the parameter.

For the two greetComedianSafety() function calls at the bottom of page 126, change their corresponding comments as the first call logs "Announcing BETTY WHITE!" and the second call logs "Well, I'm off.".

Note from the Author or Editor:
Thanks for the spot!

Change "const ratings: Ratings = { audience: 66, critic: 84 };" to "const ratings: Ratings = { audience: 66, critics: 84 };".

The change is to the "critic" property which is incorrectly declared as its non-plural form and doesn't match the Ratings interface declaration on the previous page.

Note from the Author or Editor:
Thanks for the spot!

Change all object declarations to have the plural form of the property critic.

Or you can change the property name in the Ratings interface on page 129 to match the property name critic on page 130.

Note from the Author or Editor:
Thanks for the spot!

Change "// Error: Argument of type '"missing"'..." to "// Error: Argument of type '"invalid"'...".

Note from the Author or Editor:
Thanks for the spot!

In the first sentence, "defined" should be "undefined".

Note from the Author or Editor:
Thanks for the spot!

The "maybeValue" and "knownValue" variables will be of type "number | undefined" and "number" respectively as Map.get() is retrieving the numeric value of the provided string key.

As the type returned by Map.get() in this example is "number | undefined" or just "number" with non-null assertion, the toUpperCase() method doesn't exist on the number type.

Note from the Author or Editor:
Thanks for the spot!

The "knownValue" variable will be of type "number" rather than "string" as the Map.get() method is returning the corresponding number value for the provided string key.

As "knownValue" is of type number, the toUpperCase() method wont exist on it.

Note from the Author or Editor:
Thanks for the spot!

The first error comment states { one: number; } which needs to be changed to { name: string; }.

The second error comment incorrectly states "reading 'toPrecision'" which should say "reading 'join'".

Note from the Author or Editor:
Thanks for the spot!

"LogAction" should be "Joke" instead.

Note from the Author or Editor:
Thanks for the spot!

The variable "favoritesConst" is not defined in the code example on page 139. Did you mean "preferencesReadonly" instead?

Note from the Author or Editor:
Thanks for the spot!

The usage and error message of the CrateLike<T> interface are incorrectly named.

Change the 'Crate<T>' in the error message to 'CrateLike<T>' so it matches with its interface declaration.

Also change '"inside: "??"' to "contents: "??"' so it matches the contents property declared in the interface.

Note from the Author or Editor:
Thanks for the spot!

The type of allExplicit is KeyValuePair<string, number>, not KeyValuePair<string, string>.

The type inferred by TypeScript for the variable "oneDefaulting" is "KeyValuePair<string, string>" rather than "KeyValuePair<string>".

"KeyValuePair<string, string>" is what is displayed when hovering over the "oneDefaulting" variable in VS Code.

Note from the Author or Editor:
Thanks for the spot!

The third paragraph states that lengthEventually waits an additional second to resolve with a number.

I believe this is incorrect and it instead resolves with a number immediately after textEventually resolves as the function provided to the .then() method doesn't call setTimeout().

Note from the Author or Editor:
Thanks for the spot!

The "composite" setting should be nested inside the "compilerOptions".

Note from the Author or Editor:
Thanks for the spot!

The key of the property should be "greet" instead of "log".

Note from the Author or Editor:
Thanks for the spot!

The describe() function is using "Settings.name" and "Settings.version" in its template string literal.

This is incorrect and "Settings." should be removed in both instances as the "name" and "version" value are being imported from the "./constants" module.

Note from the Author or Editor:
Thanks for the spot!

"OptionalReadonlyConservationist" should be "OptionalReadonlyEnvironmentalist" to match the type name in the code below.

Note from the Author or Editor:
Thanks for the spot!

ConditionalType<T | U> is the same as Conditional<T> | Conditional<U>
should be
ConditionalType<T | U> is the same as ConditionalType<T> | ConditionalType<U>

Note from the Author or Editor:
Thanks for the spot!

Change the word "null" to "false" in the sentence ""This OnlyStrings generic conditional type filters out types..." so it matches the union provided to OnlyStrings<> in the code below this sentence.

Note from the Author or Editor:
Nice spot, thanks!

Change "This ConfigGetter type is based..." to "This LazyValues type is based...".

Change code comment keys "location", "name", and "year" to "locationLazy", "nameLazy", and "yearLazy".

Note from the Author or Editor:
Nice spot, thanks!

Change the last sentence of the "distributivity" glossary entry to "ConditionalType<T | U> is the same as ConditionalType<T> | ConditionalType<U>".

Note from the Author or Editor:
Nice spot, thanks!