📘 How does JavaScript run in the page?

What you’ll learn

1. Values, Types, and VariablesIdentify JavaScript's primitive types and declare variables correctly using let, const, and var.JavaScript has seven primitive types plus the object type, and `typeof` reports a value's type at runtime (with the historical quirk that `typeof null` is "object"). All ordinary numbers share one IEEE 754 double-precision Number type, so there is no separate integer type. Declare bindings with `const` by default and `let` when reassignment is needed; avoid `var`, whose function scope and hoisting behavior cause subtle bugs.
2. Operators, Coercion, and EqualityPredict the result of JavaScript expressions involving type coercion and choose strict equality appropriately.JavaScript automatically converts (coerces) values between types in many expressions, which makes `+` overload between addition and string concatenation and makes loose equality (`==`) surprising. Strict equality (`===`) compares without conversion and is the default professionals use; loose equality applies a complex set of conversion rules. Truthiness rules and short-circuiting logical operators let you write concise conditionals once you know which values are falsy.
3. Control Flow and FunctionsDirect program execution with conditionals and loops and package reusable logic into functions.Control-flow statements (`if`/`else`, `switch`) and loops (`for`, `while`, `for...of`) decide which code runs and how often. Functions package logic for reuse and can be written as declarations, expressions, or arrow functions, each differing in hoisting and `this` behavior. Parameters can have default values and collect extra arguments with rest syntax, and a function's scope plus closures determine which variables it can access.
4. Arrays, Objects, and Iteration MethodsModel data with arrays and objects and transform it using core iteration methods.Arrays are ordered, zero-indexed lists and objects are unordered collections of key-value pairs; together they model most application data. Higher-order array methods like `map`, `filter`, `reduce`, and `forEach` express transformations declaratively by passing a callback. Destructuring and the spread operator let you extract and copy data concisely, and JSON provides a text format for serializing objects and arrays.
5. Building the Artifact: An Interactive DOM Mini-AppCombine selection, events, and state updates to build a small interactive web page with JavaScript.This capstone connects JavaScript to a web page through the Document Object Model (DOM), the live tree of nodes the browser builds from HTML. You select elements with `querySelector`, respond to user actions with `addEventListener`, and update the page by changing text, attributes, and classes. The lesson walks through building a tiny interactive artifact, such as a click counter or to-do list, that holds state in a variable and re-renders the DOM whenever that state changes.

Questions this course answers

Grounded in trusted sources

• MDN Web Docs, JavaScript Guide, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide
• ECMA-262 ECMAScript Language Specification — values and equality
• Marijn Haverbeke, Eloquent JavaScript — fundamentals and DOM
• You Don’t Know JS (Kyle Simpson) — types and coercion
• WHATWG DOM Standard — nodes and events overview

Every Wunder lesson is built from real, reputable sources — never invented.

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