Convert to TypeScript

.gitignore

@@ -1 +1,2 @@
-node_modules
+node_modules
+dist

.npmignore

@@ -0,0 +1 @@
+index.ts

README.md

@@ -16,7 +16,7 @@ $ npm install --save swedish-personal-identity-number-validator
 ## Usage
 
 ```js
-const isValid = require('swedish-personal-identity-number-validator');
+import { isValid } from 'swedish-personal-identity-number-validator';
 
 isValid('foobar') // => false
 ```

index.ts

@@ -0,0 +1,38 @@
+const pattern = /^\d{6}(\d{2})?[+-]?\d{4}$/;
+
+const hasCorrectChecksum = (input: string) => {
+  const sum = input
+    .split('')
+    .reverse()
+    .map(Number)
+    .map((x, i) => i % 2 ? x * 2 : x)
+    .map((x) => x > 9 ? x - 9 : x)
+    .reduce((x, y) => x + y);
+
+  return sum % 10 === 0;
+};
+
+const hasValidDate = (input: string) => {
+  let [_, yearStr, monthStr, dayStr] = /^(\d{2})(\d{2})(\d{2})/.exec(input);
+
+  const year = Number(yearStr);
+  const month = Number(monthStr) - 1;
+  const day = Number(dayStr);
+  const date = new Date(year, month, day);
+
+  const yearIsValid = String(date.getFullYear()).substr(-2) === yearStr;
+  const monthIsValid = date.getMonth() === month;
+  const dayIsValid = date.getDate() === day;
+
+  return yearIsValid && monthIsValid && dayIsValid;
+};
+
+export const isValid = (input: string) => {
+  if (!pattern.test(input)) {
+    return false;
+  }
+
+  const cleaned = input.replace(/[+-]/, '').slice(-10);
+
+  return hasCorrectChecksum(cleaned) && hasValidDate(cleaned);
+};