Not understanding the output of this forEach() method

Question

Consider this example:

var a = {
 "Check" : function doThis(value, index, ar) { 
 if(value >= 10) {
 document.write(value + ", ");
 }
 else {
 document.write("The array element is less than 10, ");
 }
 } 
}
var arr = [12, 20, 2, 3, 15];
document.write(arr.forEach(a.Check)); 

Which results in:

12, 20, The array element is less than 10, 
The array element is less than 10, 15, undefined

I don't understand why there is an extra element in the array which is undefined. Does it have something to do with defining the callback function in an object?

• millimoose: arr.forEach() doesn't have a return value, i.e. it "returns" undefined, which you're also writing. (Note the lack of a trailing comma behind it.) There is no extra element in the array, your code is buggy.
• Anon: Ahhh that makes sense - thank you! I was using document.write just for testing purposes.
• Tgr: You should use console.log instead for testing, it is vastly superior.

Answer 1

replace:

document.write(arr.forEach(a.Check)); 

with:

arr.forEach(a.Check); 

With document.write(arr.forEach(a.Check)); you are printing what the arr.forEach() call returns (undefined)