But is this possible...
Yes! This is possible. I recently wrote about it. Note that it's not specific to GitHub, just Git in general - as it's a pre-commit hook, it runs before any data is sent to GitHub.
Any appropriately-named executable files in the /.git/hooks directory of your repository will be run as hooks. There will likely be a bunch of example hooks in there already by default. Here's a simple shell script that I use as a JSLint pre-commit hook (you could modify it very easily to work with JSHint instead):
#!/bin/sh
files=$(git diff --cached --name-only --diff-filter=ACM | grep "\.js$")
if [ "$files" = "" ]; then
exit 0
fi
pass=true
echo "\nValidating JavaScript:\n"
for file in ${files}; do
result=$(jslint ${file} | grep "${file} is OK")
if [ "$result" != "" ]; then
echo "\t\033[32mJSLint Passed: ${file}\033[0m"
else
echo "\t\033[31mJSLint Failed: ${file}\033[0m"
pass=false
fi
done
echo "\nJavaScript validation complete\n"
if ! $pass; then
echo "\033[41mCOMMIT FAILED:\033[0m Your commit contains files that should pass JSLint but do not. Please fix the JSLint errors and try again.\n"
exit 1
else
echo "\033[42mCOMMIT SUCCEEDED\033[0m\n"
fi
You can simply put that in an executable file named pre-commit in your Git hooks directory, and it will run before every commit.