add CheckSnakeCase method

String/CheckSnakeCase.js

@@ -0,0 +1,20 @@
+// CheckSnakeCase method checks the given string is in snake_case or not.
+
+// Problem Source & Explanation: https://en.wikipedia.org/wiki/Naming_convention_(programming)
+
+/**
+ * CheckSnakeCase method returns true if the string in snake_case, else return the false.
+ * @param {String} varName the name of the variable to check.
+ * @returns `Boolean` return true if the string is in snake_case, else return false.
+ */
+const CheckSnakeCase = (varName) => {
+  // firstly, check that input is a string or not.
+  if (typeof varName !== 'string') {
+    return new TypeError('Argument is not a string.')
+  }
+
+  const pat = /(.*?)_([a-zA-Z])*/
+  return pat.test(varName)
+}
+
+module.exports = CheckSnakeCase