Apply the rules
Inspect a small .js example with a suitable tool. Explain its structure and one failure mode before opening untrusted files.
File Formats / Knowledge library
.js — JavaScript source
Web & source: understand the .js format, structure and safe handling.
Why learn .js — JavaScript source?
Recognize JavaScript source files and avoid incorrect parsing, unsafe loading or format assumptions.
The problem it solves
Helps select and validate JavaScript source data instead of trusting a filename extension alone.
Where it came from
Which JavaScript declaration is block-scoped? (.js) let. What does a JavaScript async function return? (.js) A Promise. What does strict equality avoid in JavaScript? (.js) Type coercion before comparison. Can a const-bound JavaScript object still have mutable properties? (.js) Yes; const prevents rebinding. What happens when JSON.parse receives invalid JSON in JavaScript? (.js) It throws a SyntaxError.
- Which JavaScript declaration is block-scoped? (.js) let.
- What does a JavaScript async function return? (.js) A Promise.
- What does strict equality avoid in JavaScript? (.js) Type coercion before comparison.
- Can a const-bound JavaScript object still have mutable properties? (.js) Yes; const prevents rebinding.
- What happens when JSON.parse receives invalid JSON in JavaScript? (.js) It throws a SyntaxError.
Build your understanding
Work through these exercises in order. Try the task before opening a reference.
- Which JavaScript declaration is block-scoped? (.js)
- What does a JavaScript async function return? (.js)
- What does strict equality avoid in JavaScript? (.js)
- Can a const-bound JavaScript object still have mutable properties? (.js)
- What happens when JSON.parse receives invalid JSON in JavaScript? (.js)
More ways to practice
Documentation and sources
Catalog research notes
- Learning notes checked October 13, 2026. Consult the official source for subsequent changes.