Fix undefined bug for template strings. (#45)

Author: beefancohen

__tests__/src/getPropLiteralValue-test.js

@@ -158,10 +158,10 @@ describe('getLiteralPropValue', () => {
       assert.equal(expected, actual);
     });
 
-    it('should drop variables in template literals that are literally undefined', () => {
+    it('should return string "undefined" for expressions that evaluate to undefined', () => {
       const prop = extractProp('<div foo={`bar ${undefined}`} />');
 
-      const expected = 'bar ';
+      const expected = 'bar undefined';
       const actual = getLiteralPropValue(prop);
 
       assert.equal(expected, actual);
@@ -178,10 +178,10 @@ describe('getLiteralPropValue', () => {
       assert.equal(expected, actual);
     });
 
-    it('should drop variables in template literals that are literally undefined', () => {
+    it('should return string "undefined" for expressions that evaluate to undefined', () => {
       const prop = extractProp('<div foo={noop`bar ${undefined}`} />');
 
-      const expected = 'bar ';
+      const expected = 'bar undefined';
       const actual = getLiteralPropValue(prop);
 
       assert.equal(expected, actual);

__tests__/src/getPropValue-test.js

@@ -194,10 +194,10 @@ describe('getPropValue', () => {
       assert.equal(expected, actual);
     });
 
-    it('should drop variables in template literals that are literally undefined', () => {
+    it('should return string "undefined" for expressions that evaluate to undefined', () => {
       const prop = extractProp('<div foo={`bar ${undefined}`} />');
 
-      const expected = 'bar ';
+      const expected = 'bar undefined';
       const actual = getPropValue(prop);
 
       assert.equal(expected, actual);
@@ -232,10 +232,10 @@ describe('getPropValue', () => {
       assert.equal(expected, actual);
     });
 
-    it('should drop variables in template literals that are literally undefined', () => {
+    it('should return string "undefined" for expressions that evaluate to undefined', () => {
       const prop = extractProp('<div foo={noop`bar ${undefined}`} />');
 
-      const expected = 'bar ';
+      const expected = 'bar undefined';
       const actual = getPropValue(prop);
 
       assert.equal(expected, actual);

src/values/expressions/TemplateLiteral.js

@@ -4,7 +4,7 @@
  * prop. For instance `This is a ${prop}` will return 'This is a {prop}'.
  *
  * If the template literal builds to undefined (`${undefined}`), then
- * this should return "".
+ * this should return "undefined".
  */
 export default function extractValueFromTemplateLiteral(value) {
   const {
@@ -20,7 +20,7 @@ export default function extractValueFromTemplateLiteral(value) {
     if (type === 'TemplateElement') {
       return raw + part.value.raw;
     } else if (type === 'Identifier') {
-      return part.name === 'undefined' ? raw : `${raw}{${part.name}}`;
+      return part.name === 'undefined' ? `${raw}${part.name}` : `${raw}{${part.name}}`;
     } else if (type.indexOf('Expression') > -1) {
       return `${raw}{${type}}`;
     }