try

array.reduce( (s,x,i) => s+(i>0 ? separator : '') + (x==null ? '' : x), '') 

function myJoin(array, separator=',') { 
  return array.reduce( (s,x,i) => s+(i>0 ? separator : '') + (x==null ? '' : x), '') 
}

console.log( myJoin(['a', 'b', 'c'], '+') ); 
console.log( myJoin(['Peter', 'Paul', 'Mary']) );
console.log( myJoin(['hello', undefined, 'world'], '-') );

We use here standard js functionalities: arrow functions, array reduce and ternary operator. If i>0 (not true only for first element) we add separator to output string s. If x is undefined or null we add to s empty string - or x value in other case.

The method array.toString() actually calls array.join() which result in a string concatenated by commas. ref

var array = ['a','b','c','d','e','f'];
document.write(array.toString()); // "a,b,c,d,e,f"

Also, you can implicitly call Array.toString() by making javascript coerce the Array to an string, like:

//will implicitly call array.toString()
str = ""+array;
str = `${array}`;

Array.prototype.join()

The join() method joins all elements of an array into a string.

Arguments:

It accepts a separator as argument, but the default is already a comma ,

str = arr.join([separator = ','])

Examples:

var array = ['A', 'B', 'C'];
var myVar1 = array.join();      // 'A,B,C'
var myVar2 = array.join(', ');  // 'A, B, C'
var myVar3 = array.join(' + '); // 'A + B + C'
var myVar4 = array.join('');    // 'ABC'

Note:

If any element of the array is undefined or null , it is treated as an empty string.

Browser support:

It is available pretty much everywhere today, since IE 5.5 (1999~2000).

References

• ECMA Specification
• Mozilla
• MSDN

The Array.prototype.join() method:

var arr = ["Zero", "One", "Two"];

document.write(arr.join(", "));