What you'll be able to do

By the end of this page you should be able to approach small practice problems with a calm process, solve a few warmup tasks, and know how to check your own answers.

Previous post: testing basics for JS functions.

Who this is for

  • People who freeze when a problem has no tutorial GIF
  • Juniors preparing for light interviews or homework drills
  • Anyone who finished syntax posts and needs reps

You can skip the specific prompts if you already drill daily — still steal the process. This opens Phase 12 — practice.

A calm process (use every time)

  1. Restate the input and output in one sentence.
  2. Do one tiny example on paper or in comments.
  3. Write the simplest correct version.
  4. Test edge cases (empty, one item, weird types if relevant).

Scenario A — "sum an array." Input: numbers. Output: one number. Empty → 0.

Scenario B — "reverse a string." Input: text. Output: text. Empty → "".

Warmup 1 — sum

function sum(nums) {
  let total = 0;
  for (let i = 0; i < nums.length; i++) total += nums[i];
  return total;
}

Scenario A — [1,2,3] → 6.

Scenario B — [] → 0.

Warmup 2 — count vowels

function countVowels(str) {
  const vowels = "aeiou";
  let count = 0;
  const lower = str.toLowerCase();
  for (let i = 0; i < lower.length; i++) {
    if (vowels.includes(lower[i])) count++;
  }
  return count;
}

Scenario A — "Hello" → 2.

Scenario B — "why" → 0 with this simple aeiou set (y rules vary — define yours).

Warmup 3 — unique values

function unique(arr) {
  const seen = new Set();
  const out = [];
  for (const item of arr) {
    if (!seen.has(item)) {
      seen.add(item);
      out.push(item);
    }
  }
  return out;
}

Scenario A — keep first-seen order.

Scenario B — [1,1,2] → [1,2].

How to practice without cheating yourself

Scenario A — peek at a solution after 20 focused minutes, then rewrite from memory the next day.

Scenario B — only read solutions. You get the illusion of skill; timed recalls fix that.

A tiny practice file

<!DOCTYPE html>
<html>
  <body>
    <pre id="out"></pre>
    <script>
      function sum(nums) {
        let total = 0;
        for (let i = 0; i < nums.length; i++) total += nums[i];
        return total;
      }
      function assertEqual(a, e, label) {
        if (a !== e) throw new Error(label + " failed");
      }
      try {
        assertEqual(sum([1, 2, 3]), 6, "sum");
        assertEqual(sum([]), 0, "empty");
        document.getElementById("out").textContent = "warmup checks passed";
      } catch (err) {
        document.getElementById("out").textContent = String(err.message);
      }
    </script>
  </body>
</html>

Add asserts for vowels and unique yourself.

Mistakes I see a lot

1. Optimizing before a correct answer exists.

2. Ignoring empty inputs.

3. Copying a clever one-liner you cannot explain in an interview.

4. Practicing only in your head — type it.

What to try before the next post

  1. Solve the three warmups without looking back.
  2. Invent one more string problem and write asserts.
  3. Timebox 25 minutes of practice three days this week.

Next in this series: mini project — todo DOM — put events, arrays, and rendering together.

Try this next outside the series

Practice goes further when you test the result and compare revisions instead of solving once and forgetting.

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