Refactor assertString: Faster, less nested and more consistent.

src/lib/util/assertString.js

@@ -1,11 +1,4 @@
 export default function assertString(input) {
-  const isString = typeof input === 'string' || input instanceof String;
-
-  if (!isString) {
-    let invalidType = typeof input;
-    if (input === null) invalidType = 'null';
-    else if (invalidType === 'object') invalidType = input.constructor.name;
-
-    throw new TypeError(`Expected a string but received a ${invalidType}`);
-  }
+  if (input === undefined) throw new TypeError(`Expected a string but received a ${input}`);
+  if (input.constructor.name !== 'String') throw new TypeError(`Expected a string but received a ${input.constructor.name}`);
 }

test/util.test.js

@@ -4,6 +4,8 @@
  */
 import assert from 'assert';
 import typeOf from '../src/lib/util/typeOf';
+import assertString from '../src/lib/util/assertString';
+
 
 describe('Util', () => {
   it('should validate different typeOf', () => {
@@ -18,3 +20,25 @@ describe('Util', () => {
     assert.notStrictEqual(typeOf([]), 'object');
   });
 });
+
+describe('assertString', () => {
+  it('Should throw an error if no argument is provided', () => {
+    assert.throws(() => { assertString(); }, TypeError);
+  });
+
+  it('Should throw an error if the argument is not a string, number', () => {
+    assert.throws(() => { assertString(123); }, TypeError);
+  });
+
+  it('Should throw an error if the argument is not a string, Object', () => {
+    assert.throws(() => { assertString({}); }, TypeError);
+  });
+
+  it('Should throw an error if the argument is not a string, Array', () => {
+    assert.throws(() => { assertString([]); }, TypeError);
+  });
+
+  it('Should not throw an error if the argument is a string', () => {
+    assert.doesNotThrow(() => { assertString('testing'); });
+  });
+});