Improve error handling

lib/calc-text-width.js

@@ -4,21 +4,23 @@ function calcWidth (charWidthTable) {
   const SCALE = 10
   const widthTable = charWidthTable.map(w => Math.round(w * SCALE))
   return function (text) {
-    if (typeof text !== 'string') {
+    typeAssert(typeof text === 'string', 'Input must be string')
-      return 0
+
-    } else {
+    let total = 0
-      let total = 0
+    let i = text.length
-      let i = text.length
+    let charCode = 0
-      let charCode = 0
+    while (i--) {
-      while (i--) {
+      charCode = text[i].charCodeAt()
-        charCode = text[i].charCodeAt()
+      total += widthTable[charCode < 127 ? charCode : 64] // "@" for overflows
-        total += widthTable[charCode < 127 ? charCode : 64] // "@" for overflows
-      }
-      return total / SCALE
     }
+    return total / SCALE
   }
 }
 
+const typeAssert = (assertion, message) => {
+  if (!assertion) throw new TypeError(message)
+}
+
 module.exports = {
   Verdana11: calcWidth(widthsVerdana11)
 }

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()
+})