## The tech lead's Slack message > *"We have 'Nexus' hardcoded in 47 places. If we rebrand tomorrow, someone has to find and change all 47. Store it in a variable. That's what variables are for."* This is the actual reason variables exist. **Not** because programming textbooks say so. Because repeating the same value everywhere is a maintenance nightmare. --- ## What is a variable? A variable is a **named container** that holds a value. Think of it like a labeled box. You write a name on the outside, put a value inside, and then use the name whenever you need the value. Watch this: Now instead of typing `"Nexus"` 47 times, you type `companyName` 47 times. And when the company rebrands to "Apex", you change **one line**. Variable = labeled box. The label (name) goes on the outside. The value goes inside. You read and write through the label. --- ## `const` — your default choice `const` creates a variable that **cannot be reassigned** after you give it a value. Use `const` for anything that shouldn't change: company names, user IDs, configuration values, API endpoints. **This is your default.** Reach for `const` first, every time. --- ## `let` — when the value needs to change `let` creates a variable that **can be reassigned**. Use it when the value will be updated — a score that goes up, a counter that increments, a status that flips. See the difference in action — click "next" to step through: Start with const. If you get an error because you need to reassign it, switch to let. You'll build the habit fast. --- ## `var` — the old way. Don't use it. `var` is how JavaScript worked before 2015. It has weird scoping rules that cause subtle bugs. You'll see it in old code and tutorials, but **never write it yourself**. Old code uses var everywhere. You don't need to fix it — just don't write new var declarations. Use const or let. --- ## Naming your variables The name you give a variable matters. A lot. Bad names make code unreadable. Good names make code self-documenting. **The rules JavaScript enforces:** - Must start with a letter, `_`, or `$` (not a number) - No spaces - No reserved keywords (`const`, `let`, `if`, etc.) **The convention JavaScript developers follow:** - `camelCase` for everything — first word lowercase, each subsequent word capitalized - Names that describe *what the value is*, not *how it's used* Click any name below to see why it's good, questionable, or broken: --- ## How JavaScript executes variable code --- ## The three things you do with variables ```js // 1. DECLARE — create the box with a name const companyName = "Nexus" // 2. READ — use the value console.log(companyName) // "Nexus" console.log("Welcome to", companyName) // "Welcome to Nexus" // 3. UPDATE — only works with let, not const let userCount = 0 userCount = userCount + 1 // now 1 userCount = userCount + 1 // now 2 console.log(userCount) // 2 ``` let status — this creates the box but leaves it empty. The value is undefined until you assign one. Only do this with let, not const (const requires a value immediately). --- ## Before the challenge Your challenge uses three variables — one for each type of data the dashboard tracks. Before you write them, think: - Should any of these change at runtime? (Probably not for this challenge — use `const`) - What names make the code readable without comments?