fix: return NaN, not 0, for an unparseable quantity ("an hour" === 0)

index.js

@@ -10,6 +10,18 @@ units.month = units.day * 30;
 units.year = units.day * 365;
 
 const regexp = /(second|minute|hour|day|week|month|year)s?/;
+const article = /^an?$/;
+
+// numbered.parse() returns 0 for anything it cannot parse, which is
+// indistinguishable from a genuine zero, so only trust 0 when the words say so.
+const parseWords = words => {
+  if (article.test(words)) {
+    return 1;
+  }
+
+  const number = numbered.parse(words);
+  return number === 0 && !/\bzero\b/.test(words) ? Number.NaN : number;
+};
 
 const humanInterval = time => {
   if (!time || typeof time === 'number') {
@@ -32,7 +44,7 @@ const humanInterval = time => {
     if (matchedNumber.length > 0) {
       number = Number.parseFloat(matchedNumber);
       if (Number.isNaN(number)) {
-        number = numbered.parse(matchedNumber);
+        number = parseWords(matchedNumber);
       }
     }
 

test.js

@@ -123,3 +123,14 @@ test('Understands mixed time expressions with singulars', macro, ['one minute an
 test('Understands 2 digit english numbers', macro, 'thirty three seconds', 33 * units.second);
 test('Understands mix units + multi digit english numbers', macro, 'hundred and three seconds and twelve minutes', (103 * units.second) + (12 * units.minute));
 test('Understands hyphenated numbers', macro, 'three hundred and twenty-five seconds', (325 * units.second));
+
+// Articles
+test('Understands "a second"', macro, 'a second', units.second);
+test('Understands "an hour"', macro, 'an hour', units.hour);
+test('Understands "a day"', macro, 'a day', units.day);
+test('Understands an article in a mixed expression', macro, 'an hour and 30 minutes', units.hour + (30 * units.minute));
+
+// Unparseable quantities are NaN, not zero
+test('Returns NaN for an unknown quantity word', macro,
+  ['a few minutes', 'some seconds', 'several hours', 'couple of hours', 'half an hour'], Number.NaN);
+test('Still understands an explicit zero', macro, 'zero seconds', 0);