TL;DR. The indexes got in this way may not be what you anticipated.

If your are usingfor(idx in arr) to enumerate through an Array, note that the type ofidx is actuallystring, notnumber.

>for(iinarr){console.log(`type of key${i}:${typeof(i)}`)}typeofkey0:stringtypeofkey1:stringtypeofkey2:stringtypeofkey3:stringtypeofkey4:string

That is, if you want to derive some values from the index in your loop like this:

for(iinarr){letval=i+1+arr2[i-1];// will probably become sth like "0122" rather than a number.}

Your code will either explode or behave unexpectedly.

This is because the indexes of an Array are actually enumerable properties of an Object and are of typestring. TheMDN doc has some explanation, and you can also check the indexes of an Array as properties byObject.getOwnPropertyNames.

>Object.getOwnPropertyNames(arr)['0','1','2','3','4','length']

Also, it is suggested that you'd better not use this to iterate through an Array if the order of execution is important since it is arbitrary according toMDN doc.

That's today's joke. Sorry if there is any misunderstanding and corrections are appreciated!.