Errata

``` 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!)

Last line of the code example is: { '1,2,3': 1 } The value should be 2, not 1: { '1,2,3': 2 }

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).

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.

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.

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.

devDependencies should include @types/react

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

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