Looping in Javascript and Typescript

Javascript and Typescript have way too many ways to iterate and loop over arrays, objects, and iterables. In this post I try to collect them and make some sense of them.

ConstructEffect on arrayEffect on objectEffect on iterable (Map, Set)
for (let i=0;
★Iterates over indicesN/AN/A
for x in objIterates over indices and custom props (including prototype props), in random order(!)★Iterates over enumerable obj props (including prototype props)Iterates over Map keys; doesn't work for Set
for x of obj★Iterates over values (in order)N/A 1★Iterates over [key, value] for Map, elements for Set
obj.forEach(x => ...)★Iterates over values (also passes index & array to callback) 2 3N/A 4★Map: iterates over values + keys 2



  1. gives this error: "Error: obj is not iterable".
  2. .forEach() has no way to break out from the middle of the loop; the others ways of iterating over an array or object support break and continue. 2
  3. arr.forEach() does not work properly with await calls in the callback, but the others support await in the body.
  4. obj.forEach() does not work on an Object, but you can do Object.keys(obj).forEach(...). That gets the keys of the object as an array, and then loops over the array of keys.