Return multiple lines instead of console.log

Push the results into an array then use the Array#join function.

function product() {
  const advertiserId = 1234;
  const ord = 543210;
  const productId = "id1|id2|id3|id4".split("|");
  const productName = "product1|product2|product3|product4".split("|");
  const productItemPrice = "10|20|30|40".split("|");

  const results = [];

  for (let i = 0; i < productId.length; i++) {
    results.push(`${advertiserId}|${ord}|${productId[i]}|${productName[i]}|${productItemPrice[i]}`);
  }

  return results;
}

const data = product();
console.log(data.join('\n'));

You could also use other console commands:

console.dir(data); 

When you return from a function, it won't continue to execute. In your case, you can store each result and return them all at once.

function product() {
    var advertiserId = 1234;
    var ord = 543210;
    var productId = "id1|id2|id3|id4".split("|"); 
    var productName = "product1|product2|product3|product4".split("|"); 
    var productItemPrice = "10|20|30|40".split("|"); 
    var results = [];
    for(i = 0; i < productId.length; i++) {
       results.push(advertiserId + "|" + ord + "|" + productId[i] + "|" + productName[i]+ "|" +productItemPrice[i]);
    }
    return results;
}

results is now an array of strings that you can do what you want with.

Write the output to a variable then return. using return inside the loop will exit the loop

function product() {
var advertiserId = 1234;
var ord = 543210;
var productId = "id1|id2|id3|id4".split("|"); 
var productName = "product1|product2|product3|product4".split("|"); 
var productItemPrice = "10|20|30|40".split("|"); 

var output = "";

    for(i = 0; i < productId.length; i++) {

    output += advertiserId+"|"+ord+"|"+productId[i]+"|"+productName[i]+"|"+productItemPrice[i] + "\n";

    }

    return output;
}
product()

I think what you need is to store every result of the iteration on a variable, and then after the for loop you return it. Like

  var a;
  for loop... { 
    a += (all of your stuff)  + '\n' 
  }
  return a;

This should keep the break lines into the variable and so keep the indentation you want like console.log but into a variable.

If you need to have separated values you could store every line into a array and return the array after the for loop.

A return statement stops the function immediately and can only return a single value. You can add up data in the loop and then return at the end.

function product() {
    var advertiserId = 1234;
    var ord = 543210;
    var productId = "id1|id2|id3|id4".split("|"); 
    var productName = "product1|product2|product3|product4".split("|"); 
    var productItemPrice = "10|20|30|40".split("|"); 

    var result = [];
    for(i = 0; i < productId.length; i++) {

        result.push(advertiserId+"|"+ord+"|"+productId[i]+"|"+productName[i]+"|"+productItemPrice[i]);



    }
    return result;
}
product()

Can map() the values into an array and return the array or array joined to string

function product() {
  var advertiserId = 1234;
  var ord = 543210;
  var productId = "id1|id2|id3|id4".split("|");
  var productName = "product1|product2|product3|product4".split("|");
  var productItemPrice = "10|20|30|40".split("|");

  return productId.map(function(id, idx) {
    return [
      advertiserId,
      ord,
      productId[idx],
      productName[idx],
      productItemPrice[idx]
    ].join('|')
  }).join('\n')


}

console.log(product());