Javascript Syntax
Now that you know what JavaScript is and how to include it in a webpage, it's time to learn how to write it.
This lesson covers variables, data types, arrays, objects, and functions — the core building blocks you'll use in every D3 chart. Packed with interactive examples and exercises.
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4 minutes read📦 Variables: const and let
Like most programming languages, JavaScript stores values in variables. You'll use two keywords to create them:
const— creates a variable that cannot be reassigned. Use this by default.let— creates a variable that can be reassigned. Use it only when you need to change the value later.
You might see
var in older code. It's the legacy way of declaring variables — don't use it.// const = constant, cannot be reassigned const name = "Alice"; console.log(name); // let = can be reassigned let score = 10; console.log(score); score = 20; // ✅ This works console.log(score); // name = "Bob"; // ❌ This would throw an error!
Note: the sandbox above shows code on the left and output on the right. The output panel is called the console — we cover it in the Developer Tools lesson.
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