Lesson 02 — Arrays for QA automation

Title: Ordered Lists, Indexing, Length, and Safe Updates

Description: Arrays are used to store values in order, such as test data rows, API results, lines from a file, or related UI elements. This lesson shows how to access and update elements using indexes, how to add items with push, and why using delete for array items is usually not a good idea.

Why it matters for QA: In tests, you often compare lists (expect(actual).toEqual(expected)). Knowing how length works, how to get the last item, and how arrays behave helps you understand bugs like unexpected array size or missing values (undefined).

How to practice: Run snippets in the browser console or Node.

1. Creating arrays

An array is an ordered list. Elements can be any type.

let empty = [];

let tags = ["smoke", "regression", "api"];
console.log(tags);

let mixed = [1, 2, "ok", "fail", null, true, false];
console.log(mixed);

2. Indexing (0-based)

First element is at index 0.

let steps = ["open", "login", "assert"];
console.log(steps[0]); // "open"
console.log(steps[1]); // "login"
console.log(steps[2]); // "assert"

3. Length and last element

let ids = ["u-100", "u-101", "u-102"];
console.log(ids.length); // 3

console.log(ids[ids.length - 1]); // "u-102"

4. Replacing and updating elements

Unlike strings, array slots are mutable—you assign a new value to a position.

let statuses = ["pending", "pending", "done"];
statuses[0] = "failed";
console.log(statuses); // ["failed", "pending", "done"]

5. Updating string elements in the array

Each element is still a string: you replace the whole element, not one character inside it.

let names = ["suite", "case", "step"];
names[0] = names[0] + "A";
names[1] = names[1] + "B";
names[2] = names[2] + "C";
console.log(names);

let labels = ["a", "b", "c"];
labels[0] += "!";
labels[1] += "!";
labels[2] += "!";
console.log(labels);

6. Increment and decrement on numeric elements

let counts = [2, 3, 4, 5];
counts[0]++;
++counts[1];
counts[2]--;
--counts[3];
console.log(counts); // [3, 4, 3, 4]

7. Growing by index

You can assign past the current end; missing indices become empty slots (sparse array).

let row = [];
row[0] = "id";
row[1] = "name";
row[2] = "role";
console.log(row);

Sparse gap (easy to create by mistake):

let letters = ["x", "y", "z"];
letters[5] = "!";
console.log(letters);
// ["x", "y", "z", empty × 2, "!"] — indices 3 and 4 are holes
console.log(letters[3]); // undefined

8. push — add to the end

Preferred way to append.

let queue = [];
queue.push("first");
queue.push("second");
queue.push("third");
console.log(queue);

9. Pitfall: using delete to remove an element by index

delete arr[i] removes the property at that index but does not shift the remaining elements. The array’s length stays the same, so you end up with a hole (an empty slot), not a shorter array.

let items = ["alpha", "beta", "gamma"];
delete items[1];
console.log(items);     // [ 'alpha', <1 empty item>, 'gamma' ]
console.log(items.length); // still 3

Better patterns for QA code:

• Remove by index and re-pack: items.splice(1, 1)
• Or build a new array: items.filter(item => item !== "gamma")

10. Finding values: indexOf and lastIndexOf

When the same value appears more than once (for example duplicate "done" in a status log), indexOf returns the first index and lastIndexOf returns the last. Both return -1 if the value is not in the array.

let statuses = ["done", "new", "running", "done"];
console.log(statuses.indexOf("done"));      // 0
console.log(statuses.lastIndexOf("done"));  // 3

QA note: Use these when you need the position of an item before splice, or to check that a string appears at least once (indexOf("error") !== -1).

11. Non-destructive ranges: slice

slice(start, end) returns a new array from start up to but not including end. The original array is unchanged—handy for taking a subset to assert on without mutating test data.

let statuses = ["done", "new", "running", "done"];
console.log(statuses.slice(0, 2)); // ["done", "new"]
console.log(statuses.slice(1, 3)); // ["new", "running"]
console.log(statuses.slice(0, 1)); // ["done"]

QA note: slice(0, 1) has length 1 (only index 0). End index is exclusive, same rule as in many APIs.

Official docs

• MDN — Array
• MDN — Array.prototype.push
• MDN — Array.prototype.indexOf
• MDN — Array.prototype.slice

Quick recap

Suggested exercises

1. Given let runs = [10, 20, 30];, log the sum of first and last element without hard-coding 30.
2. Start with [] and push three test names; then replace the middle one with "skipped".
3. Predict the output of let a = [1]; a[3] = 2; console.log(a.length, a[1]);.
4. Rewrite the delete example using splice so the array becomes ["alpha", "gamma"] with length 2.

Homework

Short tasks (about 10–15 minutes). Click a task title to reveal the prompt.