for...in vs for...of vs forEach

Model answer.

• `for...in` iterates over enumerable property keys (strings) of an object, including inherited ones. Good for plain objects; risky for arrays, because keys are strings and order or extra properties can surprise you.
• `for...of` iterates over the values of any iterable: arrays, strings, Map, Set, NodeList, generators.
• `forEach` is an array method that calls a function for each element.

const marks = [70, 85];
for (const i in marks) console.log(typeof i);   // 'string', 'string'
for (const m of marks) console.log(m);          // 70, 85
marks.forEach((m, i) => console.log(i, m));

Practical choice:

• Arrays: for...of, which supports break, continue and await inside the loop. forEach cannot be stopped early, and await inside its callback does not pause the loop.
• Plain objects: Object.entries(obj) with for...of, rather than for...in.