[
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "const total = items.reduce((acc, item) => acc + item.price, 0);",
    "explanation": "reduce walks the array carrying a running value. The 0 is the starting total; drop it and an empty array throws."
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "const unique = [...new Set(values)].sort((a, b) => a - b);",
    "explanation": "A Set discards duplicates. Spreading it back to an array allows sorting; the comparator is required or numbers sort as text."
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "export async function load(url) { const res = await fetch(url); return res.json(); }",
    "explanation": "fetch resolves as soon as headers arrive, so .json() is a second await for the body. It does not throw on a 404 - check res.ok."
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "document.querySelectorAll('.row').forEach(el => el.classList.toggle('active'));",
    "explanation": "querySelectorAll returns a static NodeList, so it will not change as you mutate the elements in it."
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "if (typeof value !== 'string' || value.length === 0) { return null; }",
    "explanation": "A guard clause: reject the bad input first and return, instead of nesting the real work inside an if."
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "content": "const { id, name = 'anonymous', ...rest } = await request.json();",
    "explanation": "Destructuring with a default and a rest property. The default applies only when the value is undefined, not when it is null."
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "content": "squares = [n * n for n in range(1, 21) if n % 3 != 0]",
    "explanation": "A list comprehension: the expression, the loop, then the filter. Faster and clearer than appending in a for loop."
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "content": "def fib(n): return n if n < 2 else fib(n - 1) + fib(n - 2)",
    "explanation": "Recursion with a base case. Correct but exponential - each call spawns two more, so it recomputes the same values repeatedly."
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "content": "with open('data.csv', encoding='utf-8') as f: rows = [line.split(',') for line in f]",
    "explanation": "with closes the file even if the body raises. Iterating a file yields lines without loading it all into memory."
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "content": "counts = {word: text.count(word) for word in set(text.split())}",
    "explanation": "A dict comprehension. Note it is quadratic: count rescans the whole string per word. collections.Counter is the right tool."
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "content": "try: value = int(raw) except ValueError: value = 0",
    "explanation": "Catching the specific exception, not a bare except - a bare one would also swallow KeyboardInterrupt."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "let mut counts: HashMap<String, usize> = HashMap::new();",
    "explanation": "mut because inserting mutates it. The type annotation is needed here since nothing yet tells the compiler what it holds."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "fn main() { println!(\"{}\", args().skip(1).collect::<Vec<_>>().join(\" \")); }",
    "explanation": "skip(1) drops the program name, which is always the first argument. Vec<_> lets the compiler infer the element type."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "let total: i64 = values.iter().filter(|v| **v > 0).sum();",
    "explanation": "iter() borrows rather than consuming. The double deref is because filter hands you a reference to the iterator's own reference."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "match result { Ok(v) => println!(\"{v}\"), Err(e) => eprintln!(\"error: {e}\") }",
    "explanation": "match on a Result must cover both arms - the compiler rejects it otherwise, which is how Rust stops an error going unhandled."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "pub struct Point { pub x: f64, pub y: f64 }",
    "explanation": "Fields are private by default in Rust, so each one needs its own pub - making the struct public does not publish its contents."
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "content": "impl Display for Token { fn fmt(&self, f: &mut Formatter) -> Result { write!(f, \"{}\", self.0) } }",
    "explanation": "Implementing Display is what makes a type usable in {}. self.0 is the first field of a tuple struct."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "C",
    "content": "int *buf = malloc(len * sizeof(int)); if (!buf) return -1;",
    "explanation": "malloc returns NULL when it fails, and checking is not optional. sizeof(int) rather than 4 keeps it correct across platforms."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "C",
    "content": "for (int i = 0; i < n; i++) { sum += a[i] * b[i]; }",
    "explanation": "A dot product. The bound is < n, not <= n; off-by-one here reads past the end of the array."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "C",
    "content": "struct node { int value; struct node *next; };",
    "explanation": "A linked-list node. It refers to its own type through a pointer, which is legal because a pointer has a known size."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "Go",
    "content": "for i, v := range items { fmt.Printf(\"%d: %v\\n\", i, v) }",
    "explanation": "range yields index and value. In Go before 1.22 the loop variable was reused each iteration, which surprised anyone capturing it in a closure."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "Go",
    "content": "if err != nil { return fmt.Errorf(\"read config: %w\", err) }",
    "explanation": "Go's error convention. %w wraps the original so errors.Is and errors.As can still see it further up."
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "Java",
    "content": "public static void main(String[] args) { System.out.println(\"ready\"); }",
    "explanation": "Java's entry point. The signature is fixed: static so it runs without an instance, and taking the command-line arguments."
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Shell",
    "content": "grep -rn \"TODO\" src/ | awk -F: '{print $1}' | sort -u",
    "explanation": "grep -n prints file:line:text, awk splits on the colon to take the filename, sort -u removes the duplicates."
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Shell",
    "content": "for f in *.log; do gzip -9 \"$f\" && echo \"packed $f\"; done",
    "explanation": "The quotes around $f matter: without them a filename containing a space becomes two arguments."
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Shell",
    "content": "export PATH=\"$HOME/.local/bin:$PATH\" && command -v rg >/dev/null || echo missing",
    "explanation": "Prepending puts your own binaries ahead of the system ones. command -v is the portable test for whether something is installed."
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Git",
    "explanation": "Finds the commit that introduced a bug by bisecting history. Give it a known-good and known-bad commit and it halves the range each step.",
    "content": "git bisect start HEAD v1.2.0 && git bisect run ./test.sh"
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Git",
    "explanation": "Who last touched each line of a range, which is how you find the person to ask rather than the person to blame.",
    "content": "git log -L 40,60:src/parser.rs --oneline"
  },
  {
    "category": "programming",
    "language": "shell",
    "attribution": "Git",
    "explanation": "Recovers commits that are no longer reachable from any branch. A hard reset does not destroy them; it just stops pointing at them.",
    "content": "git reflog --date=iso | head -20"
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "explanation": "zip pairs two sequences and stops at the shorter one. strict=True makes a length mismatch an error rather than silent truncation.",
    "content": "for name, score in zip(names, scores, strict=True): print(f'{name}: {score:>5.1f}')"
  },
  {
    "category": "programming",
    "language": "python",
    "attribution": "Python",
    "explanation": "defaultdict removes the exists-check before every append, which is the most common shape of a grouping loop.",
    "content": "groups = defaultdict(list); [groups[w[0]].append(w) for w in words]"
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "explanation": "?? falls back only on null or undefined, unlike ||, which also fires on 0 and the empty string.",
    "content": "const port = config.port ?? process.env.PORT ?? 8080;"
  },
  {
    "category": "programming",
    "language": "javascript",
    "attribution": "JavaScript",
    "explanation": "Promise.all runs them concurrently and rejects on the first failure; allSettled waits for every result instead.",
    "content": "const [user, posts] = await Promise.all([getUser(id), getPosts(id)]);"
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "explanation": "The ? operator returns early on Err, converting the error type through From. It is what keeps error handling from nesting.",
    "content": "let config: Config = toml::from_str(&fs::read_to_string(path)?)?;"
  },
  {
    "category": "programming",
    "language": "rust",
    "attribution": "Rust",
    "explanation": "entry().or_insert_with() looks the key up once. The obvious contains_key-then-insert hashes it twice.",
    "content": "*counts.entry(word).or_insert(0) += 1;"
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "SQL",
    "explanation": "A window function ranks rows within each partition without collapsing them, which a GROUP BY cannot do.",
    "content": "SELECT user_id, wpm, RANK() OVER (PARTITION BY mode ORDER BY wpm DESC) AS rank FROM results;"
  },
  {
    "category": "programming",
    "language": "clike",
    "attribution": "SQL",
    "explanation": "EXPLAIN ANALYZE runs the query and reports what the planner actually did, not what it guessed it would do.",
    "content": "EXPLAIN ANALYZE SELECT * FROM test_results WHERE mode_key = 'time-30' ORDER BY wpm DESC LIMIT 50;"
  }
]