Errata

PDF is First Edition, Third Release (2021-07-16).

The text says, "If you changed to const enum Flavor in the previous example, the compiler would rewrite Flavor.CHOCOLATE as 0." However I'm pretty sure the value should be "1" instead of "0".

Note from the Author or Editor:
Should read "the compiler would rewrite Flavor.CHOCOLATE as 1."

const geometryHelper = (g: Geometry) => {
if (geometry.type === 'GeometryCollection') {
...

the parameter defined as `g` but used as `geometry`

Note from the Author or Editor:
`geometry` should be `g` inside the body of `geometryHelper`. JavaScript scoping: always confusing, always error prone!

```
function parseCSV(input: string): {[columnName: string]: string}[] {
const lines = input.split('\n');
const [header, ...rows] = lines;
return rows.map(rowStr => {
const row: {[columnName: string]: string} = {};
rowStr.split(',').forEach((cell, i) => {
row[header[i]] = cell;
});
return row;
});
}
```

I believe this code snippet does not do what the author intended (mapping column names to values), as when `header[i]` is accessed, `header` is a string consisting of the entire header row. So given the following code:

```
console.log(parseCSV(`name,age
Will,23`))
```

The output will be:

```
[{
'n': 'Will',
'a': 23,
}]
```

Rather than what I suspect the author intended:

```
[{
'name': 'Will',
'age': 23,
}]
```

Fixed by:

```
function parseCSV(input: string): {[columnName: string]: string}[] {
const lines = input.split('\n');
const [header, ...rows] = lines;
const headerColumns = header.split(',');
return rows.map(rowStr => {
const row: {[columnName: string]: string} = {};
rowStr.split(',').forEach((cell, i) => {
row[headerColumns[i]] = cell;
});
return row;
});
}
```

Very minor!

(apologies for not listing page number: O'Reilly web viewer does not give an option to display them afaik)

Note from the Author or Editor:
Yes, this is a mistake. The header row needs to be split before being used for column names. (This is why you need tests, not just types!)

`npm install --sav-dev ...` should be `npm install --save-dev ...`.

The code example is:

function pluck(records, key) {
return record.map(record => record[key]);
}

The argument is `records` in plural, while the variable that is mapped over is the undefined `record`, which also is shadowed in the arrow function. A working version would be:

function pluck(records, key) {
return records.map(record => record[key]);
}

To align with the following examples, it could be changed to:

function pluck(record, key) {
return record.map(r => r[key]);
}

Though I'd argue that the argument is an array, it should be named `records`, not `record` throughout all the examples.

Note from the Author or Editor:
They should all be `records` not `record`. I've made the fix and this will appear in the next release.

To illustrate the "chain" concept in Lodash/Underscore, two code examples are shown, intended to have an equivalent effect.

The traditional version:

_.a(_.b(_.c(v)))

The chained version:

_(v).a().b().c().value()

For these to be equivalent, the order of a/b/c must be switched in one of the examples, e.g.:

_.c(_.b(_.a(v))) and _(v).a().b().c().value()

or:

_.a(_.b(_.c(v))) and _(v).c().b().a().value()

Note from the Author or Editor:
The intent was

_.c(_.b(_.a(v))) and _(v).a().b().c().value()

I've fixed this in the book repo.

Line 6 is:

pt.y < x[0] || pt.y > y[1]) {

Instead of comparing with the lower x boundary, it should compare with the lower y boundary:

pt.y < y[0] || pt.y > y[1]) {

Note from the Author or Editor:
This has been fixed in the second release.

Line 4 is:

pt.y < polygon.bbox.y[1] || pt.y > polygon.bbox.y[1]) {

The index in the lower bound check should be 0, not 1:

pt.y < polygon.bbox.y[0] || pt.y > polygon.bbox.y[1]) {

Note from the Author or Editor:
This has been fixed in the second release (March 2020).

Last line of the code example is:

{ '1,2,3': 1 }

The value should be 2, not 1:

{ '1,2,3': 2 }

devDependencies should include @types/react

Note from the Author or Editor:
@types/lodash should be @types/react. Thanks for catching this mistake!