Improve error handling

lib/calc-text-width.js

@@ -4,9 +4,8 @@ function calcWidth (charWidthTable) {
   const SCALE = 10
   const widthTable = charWidthTable.map(w => Math.round(w * SCALE))
   return function (text) {
-    if (typeof text !== 'string') {
-      return 0
-    } else {
+    typeAssert(typeof text === 'string', 'Input must be string')
+
     let total = 0
     let i = text.length
     let charCode = 0
@@ -17,6 +16,9 @@ function calcWidth (charWidthTable) {
     return total / SCALE
   }
 }
+
+const typeAssert = (assertion, message) => {
+  if (!assertion) throw new TypeError(message)
 }
 
 module.exports = {

test/calc-text-width.spec.js

@@ -7,3 +7,8 @@ tap.test('calc width for "npm"', t => {
   t.is(calcWidth('npm'), 24.9, 'result is correct value')
   t.end()
 })
+
+tap.test('exception handling', t => {
+  t.throws(() => calcWidth(0), TypeError, 'throw if feed with non-string input')
+  t.end()
+})