JavaScript: add behavior and state

JavaScript responds to events, transforms data, updates state, and reflects state back into the interface. Keeping a single source of truth prevents the screen and underlying data from disagreeing.

At a glance

Question	Practical answer
When is it useful?	A task list stores an array of tasks; adding, completing, and filtering produce new state, then one render step updates the visible list and count.
What should you do?	Build a task list with add, complete, filter, and empty-state behavior; persist only non-sensitive data if you add local storage.
How do you know it worked?	The item count always matches visible state, keyboard and mouse actions work, and refreshing restores only the data you intentionally persisted.
Common failure	Avoid scattering state across DOM text, global variables, and storage; decide which representation is authoritative.

flowchart LR
  A[Question] --> B[JavaScript: add behavior and state]
  B --> C[Small example]
  C --> D[Evidence]

The important idea is not to stop at a definition: connect the concept to a small example and observable evidence.

Worked example

A task list stores an array of tasks; adding, completing, and filtering produce new state, then one render step updates the visible list and count.

Before acting, write the success signal. Change one condition at a time, observe the result, and record assumptions. For JavaScript: add behavior and state, this separates what you know from what you are merely guessing.

Practice in 20–30 minutes

Goal: Build a task list with add, complete, filter, and empty-state behavior; persist only non-sensitive data if you add local storage.

1. Record the starting state and your prediction.
2. Implement the smallest version without adding unnecessary tools.
3. Change exactly one input or constraint and repeat.
4. Save a command, screenshot, output, or checklist as evidence.

Expected result: The item count always matches visible state, keyboard and mouse actions work, and refreshing restores only the data you intentionally persisted.

What can go wrong

Avoid scattering state across DOM text, global variables, and storage; decide which representation is authoritative.

When the result differs from your prediction, do not change many things at once. Check inputs, versions, environment, permissions, and logs, then repeat from the smallest example.

Definition of done

• I can explain the concept in my own words.
• I completed the small example and kept evidence.
• I know one failure mode and how to check it.
• Someone else can repeat the work without guessing missing steps.

Go deeper

Use the linked resource or repository at the end of the page when you need a full implementation. Check current versions before applying commands to a real project.