Convert an array to a string

Using the toString () method could achieve this result, (response adapted to your comment code).

var array1= [100,200,300,400,500,600,700,800,900];
var array2=[1,2,5];
var posit = array2.map(function(el){
	return array1[el];
});


console.log(posit.toString()); // uso del método

Explained the question, you can do it with map , as you said @DevJoel.

And, for a array I think it's better to use join than toString .

The method join() links all the elements of a Fix in a chain. The default separator is comma , , and it offers you the possibility of using another separator if necessary. Also, if an element is undefined or null is converted to an empty string.

For other separators, see examples in the documentation link.

Here I show a simplified form:

var act = [10002197,10001755,10001087,10001879,3508477478,10001881];
var posiciones = [0,2,3] // el debera imprimir (10002197,10001087,10001879)

var arrResultado = posiciones.map(i => act[i]);
document.write(arrResultado.join());

the best and fastest option is

array = [1,2,3,5]
console.log(array.join())

will return the elements of the array in a string separated by a comma, if you want a different separation specified within the join " join(";") "