Add Check For An Executable as a vim til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_547 TILs and counting..._
+_548 TILs and counting..._
 
 ---
 
@@ -544,6 +544,7 @@ _547 TILs and counting..._
 - [Case-Aware Substitution With vim-abolish](vim/case-aware-substitution-with-vim-abolish.md)
 - [Case-Insensitive Substitution](vim/case-insensitive-substitution.md)
 - [Center The Cursor](vim/center-the-cursor.md)
+- [Check For An Executable](vim/check-for-an-executable.md)
 - [Check Your Current Color Scheme](vim/check-your-current-color-scheme.md)
 - [Close All Other Splits](vim/close-all-other-splits.md)
 - [Close All Other Windows](vim/close-all-other-windows.md)

vim/check-for-an-executable.md

@@ -0,0 +1,24 @@
+# Check For An Executable
+
+Sometimes Vim needs to reach outside of itself to use an existing program.
+For example, wiring up [auto-formatting of JavaScript
+code](https://hashrocket.com/blog/posts/writing-prettier-javascript-in-vim)
+requires Vim to call out to the
+[`prettier`](https://github.com/prettier/prettier) binary.
+
+We want our `.vimrc` files and plugins to, generally, be as portable as
+possible. What happens if you haven't yet installed a particular program?
+Vim will likely experience a runtime exception. One way to get around this
+is to check for the presence of that program on the path. If it isn't there,
+don't do the thing. We can use the `executable()` function for this.
+
+```vimscript
+if executable('prettier')
+  ...
+endif
+```
+
+It will return `1` (true) if `prettier` is an executable on the path,
+otherwise it will return `0` (false).
+
+See `:help executable()` for more details.