JavaScript Concepts

JavaScript has a small set of built-in types, and it converts between them automatically when an operator expects a different one.

When an expression mixes operators, precedence decides which one runs first, and associativity decides the order among operators of equal precedence.

let and const are block-scoped and unreachable until their line runs, while var is function-scoped and available as undefined from the top.

Every value is truthy unless it is one of a short, memorizable list of falsy values.

JavaScript sets up all your declarations before it runs a single line of your code.

A variable lookup walks outward through nested scopes until it finds a match or runs out of places to look.

this is not fixed to a function; it is decided fresh by how that function gets called.

Every function call gets its own frame on a stack, and JavaScript only ever runs the frame on top.

Copying a number copies the value; copying an object copies a pointer to the same shared value.

Destructuring unpacks values from an array or an object straight into named variables.

The same ... syntax expands a value out in one context and gathers values together in the other.

?. stops a property lookup safely at the first null or undefined instead of throwing; ?? picks a default only for null or undefined.

A Set stores unique values with no duplicates; a Map stores key-value pairs with keys of any type and fast lookup.

A function keeps access to the variables it was born in, even after that scope returns.

A function that takes another function in, gives one back out, or both.

Turning a multi-argument function into a chain of one-argument functions.

Three ways to control what this refers to when a function runs.

map transforms every element, filter keeps a subset, and reduce folds everything into one value.

Array.prototype.sort reorders elements in place, using a comparator's sign to decide each swap.

When an object does not have a property itself, JavaScript looks for it on the object's prototype instead.

A class is syntax on top of a constructor function and its prototype, not a new kind of object.

A getter runs code when a property is read, and a setter runs code when a property is assigned.

The engine finishes the current script, drains every pending microtask, then runs one task from the macrotask queue, on repeat.

A promise is a placeholder for a value that starts pending and settles exactly once, to fulfilled or rejected.

await pauses an async function until a promise settles, without blocking the rest of the program.

Microtasks, like promise callbacks, always run before the next macrotask, like a setTimeout, even a zero-delay one.

Each file is its own module with its own scope, and export/import wire live values between files, not copies.

A pattern language for finding, testing, and pulling pieces out of text.

A generator writes a pausable, resumable function that hands out one value at a time.