Why doesn't the forEach( ) method work?

Question

JavaScript

var bisOne = [1,2,3,4,5].forEach(function(e,i,arr){return arr[i]=e*10}); 
var ghost = [1,2,3,4,5]
var bisTwo = ghost.forEach(function(e,i,arr){return arr[i]=e*10}) 

console.log(bisOne);  //(1)
console.log(ghost);
console.log(bisTwo);  //(2)

var gsp=[1,2,3,4,5].reduce(function(a,b){return a+b}) 
console.log(gsp);  //(3)

I can’t understand why calls ( 1 )and ( 2 )those marked with comments in the code return undefined? After all, if you look at the method reducecall in the call under the number ( 3 ), the construction is similar, BUT working, unlike the first two! In fact, we write in bisOneand bisTwo! Please explain why and how it does NOT work!

Answer 1

Because, all of a sudden, forEach is not the same as reduce . The forEach method is used exclusively for iteration , while reduce is used to get some result , so forEach does not return anything .
You need a map method :
If you really want to do this with forEach, then it should be like this:

var bisOne = [1,2,3,4,5];
bisOne.forEach(function(e,i,arr){arr[i]=e*10});