add CheckFlatCase method

String/CheckFlatCase.js

@@ -0,0 +1,20 @@
+// CheckFlatCase method checks the given string is in flatcase or not.
+
+// Problem Source & Explanation: https://en.wikipedia.org/wiki/Naming_convention_(programming)
+
+/**
+ * CheckFlatCase method returns true if the string in flatcase, else return the false.
+ * @param {String} varname the name of the variable to check.
+ * @returns `Boolean` return true if the string is in flatcase, else return false.
+ */
+const CheckFlatCase = (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-z]*$/
+  return pat.test(varname)
+}
+
+module.exports = CheckFlatCase