add lazy evaluation

Author: Luke Westby

README.md

@@ -10,6 +10,7 @@ renderIf(predicate)(element)
 ## What it looks like
 
 `renderIf` is a curried function that takes a predicate and returns a function accepting an element that will only be returned if the predicate is satisfied.
+The function returned by `renderIf` will also accept a parameterless function which will only be invoked if the predicate is satisfied, allowing for lazy evaluation of inner JSX.
 
 ```js
 renderIf(1 + 1 === 2)(
@@ -31,6 +32,20 @@ class MyComponent extends Component {
 }
 ```
 
+### As a lazy in-line expression
+
+```jsx
+class MyComponent extends Component {
+  render() {
+    return (
+      {renderIf(1 + 2 === 3)(() => (
+        <span>This is only invoked if the universe is working</span>
+      ))}
+    );
+  }
+}
+```
+
 ### As a named function
 
 ```jsx

renderIf.js

@@ -1,3 +1,6 @@
 'use strict';
 
-export default predicate => element => predicate && element;
+const isFunction = input => typeof input === 'function';
+
+export default predicate => elemOrThunk =>
+  predicate ? (isFunction(elemOrThunk) ? elemOrThunk() : elemOrThunk) : null;

test.js

@@ -4,13 +4,23 @@ import {expect} from 'chai';
 import renderIf from './renderIf';
 
 describe('renderIf', () => {
-  it ('should return a function', () => {
+  it('should return a function', () => {
     expect(typeof renderIf()).to.be.eql('function');
   });
-  it ('should return the element when the predicate passes', () => {
-    expect(renderIf(true)('foobar')).to.be.eql('foobar');
+  describe('non-lazy', () => {
+    it('should return the element when the predicate passes', () => {
+      expect(renderIf(true)('foobar')).to.be.eql('foobar');
+    });
+    it('should not return the element when the predicate fails', () => {
+      expect(renderIf(false)('foobar')).to.be.eql(null);
+    });
   });
-  it ('should not return the element when the predicate fails', () => {
-    expect(renderIf(false)('foobar')).to.be.eql(false);
+  describe('lazy', () => {
+    it('should return the result of the thunk when the predicate passes', () => {
+      expect(renderIf(true)(() => 'foobar')).to.be.eql('foobar');
+    });
+    it('should not return the result of the thunk when the predicate fails', () => {
+      expect(renderIf(false)(() => 'foobar')).to.be.eql(null);
+    });
   });
 });