hardPythonkeyword-only args
What will this code output?
def f(x, y, /, z=10):
return x + y + z
print(f(1, 2))3,960 hard coding interview questions
Hard questions live in the specification: evaluation order, memory and lifetime, coercion corners, and the behaviour that only shows up under concurrency.
Aimed at senior screens and at anyone who wants to stop being surprised by their own language.
Expect snippets that look wrong but are correct, or look correct and are not. Reading the explanation matters more here than getting it right.
Hard questions by language
Sample hard questions
Eighteen real hard questions from across the library, code snippet included. The answer options and explanations come after you attempt them.
hardPythonmutable default trap
What happens here?
from dataclasses import dataclass
@dataclass
class A:
items: list = []
a1 = A()
a2 = A()
a1.items.append(1)
print(a2.items)hardPythonexception chaining
What happens here?
try:
raise ValueError("oops")
except Exception:
print("General")
except ValueError:
print("Value")hardPythondata model dunder dispatch
What does this code print?
class A:\n def __call__(self): return 2\nprint(A()())hardTypeScriptarrays and tuples
What is the type of x?
const arr: number[] = [1,2,3];\nconst x = arr[0]; // with noUncheckedIndexedAccesshardTypeScripttemplate literal types
What is X?
type X = `${Capitalize<"foo">}Bar`;hardTypeScriptprimitive types
What is X?
type X = string & number;hardTypeScripttypeof type queries
What is the type of f?
const f = (x: number) => x + 1;hardPythondata model dunder dispatch
What does this code print?
class A:\n v = "x"\na = A()\na.v = "y"\nprint(a.v)hardJavaScriptToPrimitive coercion algorithm
What is the output?
console.log([] == false);hardReactuseSyncExternalStore
Why can a useState-based subscription to an external store (the pre-18 pattern of setState in an effect) silently MISS a store mutation that happens between the initial render and the effect running, in a way useSyncExternalStore does not?
// legacy pattern
const [snap, setSnap] = useState(store.getSnapshot());
useEffect(() => {
const unsub = store.subscribe(() => setSnap(store.getSnapshot()));
return unsub;
}, []);hardPythoninterning
What does this code print?
a, b = 256, 256\nc, d = 257, 257\nprint(a is b, c is d)hardJavaScriptthis in callbacks vs methods
What does this code log?
const o = { x: "b", f() { return this.x; } };\nconsole.log(o.f());hardJavaScriptlet const var and scope
What does this code throw?
const x = 1;\nx = 2;hardPythonint float bool str
What does this code print?
print("hello" + 5)hardPythonvariables and None
What does this code print?
print("".join(print("hi")))hardJavaScriptgetters and setters
What does this code log?
class A { #x = 1; getX() { return this.#x; } }\nconsole.log(new A().getX());hardTypeScriptconditional-type distribution
What is X?
type X = Exclude<"a"|"b","a"|"b">;