Throw descriptive TypeError when obj is not JSON-serializable

.gitignore

@@ -1,4 +1,5 @@
 node_modules/
 /.project
 .vscode/
+package-lock.json
 

test/write-file-sync.test.js

@@ -116,4 +116,30 @@ describe('+ writeFileSync()', () => {
       done()
     })
   })
+
+  describe('> when obj is not serializable', () => {
+    it('should throw a TypeError when obj is undefined', () => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      assert.throws(
+        () => jf.writeFileSync(file, undefined),
+        { name: 'TypeError', message: 'Converting undefined value to JSON is not supported' }
+      )
+    })
+
+    it('should throw a TypeError when obj is a function', () => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      assert.throws(
+        () => jf.writeFileSync(file, function () {}),
+        { name: 'TypeError', message: 'Converting function value to JSON is not supported' }
+      )
+    })
+
+    it('should throw a TypeError when obj is a Symbol', () => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      assert.throws(
+        () => jf.writeFileSync(file, Symbol('test')),
+        { name: 'TypeError', message: 'Converting symbol value to JSON is not supported' }
+      )
+    })
+  })
 })

test/write-file.test.js

@@ -257,4 +257,44 @@ describe('+ writeFile()', () => {
         })
     })
   })
+
+  describe('> when obj is not serializable', () => {
+    it('should reject the promise with a TypeError when obj is undefined', (done) => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      jf.writeFile(file, undefined)
+        .catch(err => {
+          assert(err instanceof TypeError)
+          assert.strictEqual(err.message, 'Converting undefined value to JSON is not supported')
+          done()
+        })
+    })
+
+    it('should pass a TypeError to callback when obj is undefined', (done) => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      jf.writeFile(file, undefined, (err) => {
+        assert(err instanceof TypeError)
+        assert.strictEqual(err.message, 'Converting undefined value to JSON is not supported')
+        done()
+      })
+    })
+
+    it('should pass a TypeError to callback when obj is a function', (done) => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      jf.writeFile(file, function () {}, (err) => {
+        assert(err instanceof TypeError)
+        assert.strictEqual(err.message, 'Converting function value to JSON is not supported')
+        done()
+      })
+    })
+
+    it('should reject the promise with a TypeError when obj is a Symbol', (done) => {
+      const file = path.join(TEST_DIR, 'somefile.json')
+      jf.writeFile(file, Symbol('test'))
+        .catch(err => {
+          assert(err instanceof TypeError)
+          assert.strictEqual(err.message, 'Converting symbol value to JSON is not supported')
+          done()
+        })
+    })
+  })
 })

utils.js

@@ -2,6 +2,10 @@ function stringify (obj, { EOL = '\n', finalEOL = true, replacer = null, spaces
   const EOF = finalEOL ? EOL : ''
   const str = JSON.stringify(obj, replacer, spaces)
 
+  if (str === undefined) {
+    throw new TypeError(`Converting ${typeof obj} value to JSON is not supported`)
+  }
+
   return str.replace(/\n/g, EOL) + EOF
 }