update query selector null checking

README.md

@@ -17,8 +17,8 @@ Frontend environments evolve rapidly nowadays, modern browsers have already impl
 
 In place of common selectors like class, id or attribute we can use `document.querySelector` or `document.querySelectorAll` for substitution. The differences lie in:
 * `document.querySelector` returns the first matched element
-* `document.querySelectorAll` returns all matched elements as NodeList. It can be converted to Array using `[].slice.call`
-* If no elements matched, jQuery would return `[]` whereas the DOM API will return `null`. Pay attention to Null Pointer Exception.
+* `document.querySelectorAll` returns all matched elements as NodeList. It can be converted to Array using `[].slice.call(document.querySelectorAll(selector) || []);`
+* If no elements matched, jQuery would return `[]` whereas the DOM API will return `null`. Pay attention to Null Pointer Exception. You can also use `||` to set default value if not found, like `document.querySelectorAll(selector) || []`
 
 > Notice: `document.querySelector` and `document.querySelectorAll` are quite **SLOW**, try to use `getElementById`, `document.getElementsByClassName` or `document.getElementsByTagName` if you want to get a performance bonus.