Types and .flowconfig changes

chapter02/.flowconfig

@@ -5,7 +5,10 @@
 [libs]
 
 [lints]
+all=warn
+unsafe-getters-setters=off
 
 [options]
+include_warnings=true
 
 [strict]

chapter02/src/module_conversion_usage.js

@@ -0,0 +1,14 @@
+/* @flow */
+
+import {
+    milesToKm,
+    ouncesToGrams,
+    poundsToKg as p_to_kg
+} from "./module_conversions.js";
+
+console.log(`A miss is as good as ${milesToKm(1)} kilometers.`);
+
+console.log(
+    `${ouncesToGrams(1)} grams of protection `,
+    `are worth ${p_to_kg(1) * 1000} grams of cure.`
+);

chapter02/src/module_conversions.js

@@ -0,0 +1,26 @@
+/* @flow */
+
+type conversion = number => number;
+
+const SPEED_OF_LIGHT_IN_VACUUM_IN_MPS = 186282;
+const KILOMETERS_PER_MILE = 1.60934;
+const GRAMS_PER_POUND = 453.592;
+const GRAMS_PER_OUNCE = 28.3495;
+
+const milesToKm: conversion = m => m / KILOMETERS_PER_MILE;
+const kmToMiles: conversion = k => k * KILOMETERS_PER_MILE;
+
+const poundsToKg: conversion = p => p / (GRAMS_PER_POUND / 1000);
+const kgToPounds: conversion = k => k * GRAMS_PER_POUND / 1000;
+
+const gramsToOunces: conversion = g => g / GRAMS_PER_OUNCE;
+const ouncesToGrams: conversion = o => o * GRAMS_PER_OUNCE;
+
+/*
+    It's usually preferred to include all "export"
+    statements together, at the end of the file.
+    You need not have a SINGLE export, however.
+*/
+export { milesToKm, kmToMiles };
+export { poundsToKg, kgToPounds, gramsToOunces, ouncesToGrams };
+export { SPEED_OF_LIGHT_IN_VACUUM_IN_MPS };

chapter02/src/types_advanced.js

@@ -20,7 +20,7 @@ class Person {
     first: string;
     last: string;
 
-    constructor(first, last) {
+    constructor(first: string, last: string) {
         this.first = first;
         this.last = last;
     }