feat: refactor and test tools steps utilities (#42693)

tools/challenge-helper-scripts/helpers/get-existing-step-nums.js

@@ -0,0 +1,30 @@
+const fs = require('fs');
+const path = require('path');
+
+// Generates an array with the output of processing filenames with an expected
+// format (`part-###.md`).
+// ['part-001.md', 'part-002.md'] => [1, 2]
+function getExistingStepNums(projectPath) {
+  return fs.readdirSync(projectPath).reduce((stepNums, fileName) => {
+    if (
+      path.extname(fileName).toLowerCase() === '.md' &&
+      !fileName.endsWith('final.md')
+    ) {
+      let stepNum = fileName.split('.')[0].split('-')[1];
+
+      if (!/^\d{3}$/.test(stepNum)) {
+        throw (
+          `Step not created. File ${fileName} has a step number containing non-digits.` +
+          ' Please run reorder-steps script first.'
+        );
+      }
+
+      stepNum = parseInt(stepNum, 10);
+      stepNums.push(stepNum);
+    }
+
+    return stepNums;
+  }, []);
+}
+
+exports.getExistingStepNums = getExistingStepNums;