01 — Running JavaScript

One line: What a line of JavaScript does depends on how it was run — script or module, strict or sloppy, and on which Node — so the first skill is knowing which of those you are in.

This chapter separates the language from the program that runs it. It starts with what ECMAScript defines and what Node adds, then the ways to run code with node. Next come the rules that decide what a line means — script, CommonJS module or ES module, sloppy or strict — and automatic semicolon insertion, which decides where a line ends. It closes with versions: how a feature enters the yearly edition, and how to check that your Node has it before you rely on it.

The pages

Page	Level	In one line	Status
The language and the host	101	ECMAScript defines Array, JSON and Promise but not setTimeout, process or fetch, which Node adds; a bare node:vm context has the first three and none of the others, and the console it does have prints nothing.	stub
Running a file with Node	101	node -e '1 + 1' prints nothing and node -p '1 + 1' prints 2; node --check only parses, so it passes a file that calls a function that does not exist, and fails only on a syntax error.	stub
Scripts and modules	201	The same line prints object 1 false as a script and undefined undefined true as a module: a module is always strict, its top-level this is undefined, and its var never becomes a property of globalThis.	stub
Strict mode	201	Without "use strict", assigning to a misspelled variable creates a global and writing to a frozen object does nothing; strict mode, which every module and class body gets automatically, turns the first into a ReferenceError and the second into a TypeError.	stub
Automatic semicolon insertion	201	A line break ends a statement only when the next line cannot continue it, but always ends a return: return followed by a newline returns undefined, a line starting with ( calls the value above it, and one starting with [ indexes into it.	stub
ECMAScript versions	101	Since ES2015 an edition comes out every year, named for the year, holding the proposals that reached Stage 4 in time; engines ship in their own order, so Node 25.2.1 has ES2027's DisposableStack but not ES2026's Math.sumPrecise.	stub
Which features your Node has	201	A newer Node is not a superset of an older one: node --harmony-temporal gives Node 24 a working Temporal but leaves it undefined in Node 25.2.1, so test for a feature with typeof rather than trusting a version number.	stub

Boundaries

Module systems, import and package.json belong to chapter 14; TypeScript's own strict flag and running .ts files belong to chapter 22.