Although a script can do this easily, I recommend using regular spreadsheet formulas, e.g.

=transpose(split(join(";";Sheet1!A:A)&";"&join(";";Sheet2!A:A);";"))

To remove duplicates, just wrap it in a unique formula:

=unique(transpose(...))

And to sort... =sort(...)

You can merge them into 1 column then get the unique values. Check the following formula:

=UNIQUE({Sheet1!A:A;Sheet2!A:A})

Just use:

={sheet1!a:a; sheet2!a:a}

Google Apps Script

And yet the question was about the script. I'm still successfully using the following code:

function unionRanges(e) {
var result = [];
var length = 0;
var i = 0;
try {
for (i = 0; i < arguments.length; i++)
length += arguments[i].length;
if (length > 3000) return '#BIGRANGE';
for (var i = 0; i < arguments.length; i++)
result = result.concat(arguments[i].filter(function (el) {
return el.join('').length > 0
}));
return result;
} catch (err) {
return JSON.stringify(err);
}
}

Spreadsheets feature

But, as noted above, it is easier to use {}-notation.

={ Sheet1!A1:C10 ; Sheet2!A1:C34 }

Vertical concatenation

={ Range(Cols=N) ; Range(Cols=N) }

Horizontal concatenation

={ Range(Rows=M) , Range(Rows=M) }

It's possible to combine

={ { , , } ; { , , } }

Or something more hard

=\{\{{;;},{;;}};\{\{;;},{;;}};\{\{;;},{;;}}}

Try something like this

={
\{\{ 1; 2; 3},{ 4; 5; 6}};
\{\{ 7; 8; 9},{10;11;12}};
\{\{13;14;15},{16;17;18}}
}

The internal horizontal concatenation is not required

={
{ 1; 2; 3},{ 4; 5; 6};
{ 7; 8; 9},{10;11;12};
{13;14;15},{16;17;18}
}

Locale dependencies of argument delimiters

If your current locale supports , as an argument delimiter thnen you should use ; for a vertical concatenation and , for a horizontal concatenation.

Otherwise your argument delimiter is ; and you have to use ; and \ (without spaces), respectively.

Sheet 'Data 1'!A1:C20

|   Name  |    Date   | Sum |
| Ethan   |  3/4/2017 |  31 |
| Logan   |  3/6/2017 |  62 |
| Brian   | 3/26/2017 |  61 |
|   ...   |     ...   | ... |

Sheet 'Data 2'!A1:C20

|  Name   |    Date   | Sum |
| Nathan  | 3/30/2017 |  53 |
| Alyssa  | 3/13/2017 |  72 |
| John    | 3/24/2017 |  79 |
| Megan   | 3/16/2017 |  10 |
|   ...   |     ...   | ... |

Concatenation

Vertical concatenation

={'Data 1'!A1:C20;'Data 2'!A2:C20}

Result

|  Name  |    Date   | Sum |
| Ethan  |  3/4/2017 |  31 |
| Logan  |  3/6/2017 |  62 |
| Brian  | 3/26/2017 |  61 |
| ...    |       ... | ... |
| Nathan | 3/30/2017 |  53 |
| Alyssa | 3/13/2017 |  72 |
| John   | 3/24/2017 |  79 |
| ...    |       ... | ... |

Horizontal concatenation

={TRANSPOSE('Data 1'!A1:C20),TRANSPOSE('Data 2'!A2:C20)}

Result

| Name |   Ethan  |   Logan  |   Brian   | ... |   Nathan  |   Alyssa  |    John   |
| Date | 3/4/2017 | 3/6/2017 | 3/26/2017 | ... | 3/30/2017 | 3/13/2017 | 3/24/2017 |
| Sum  |       31 |       62 |        61 | ... |        53 |        72 |        79 |

More about this How to concatenate ranges in Google spreadsheets

If you want to union sheets and exclude rows with empty cells use the FILTER function in your formula:

=FILTER({Sheet1!A:A;Sheet2!A:A}, {Sheet1!A:A;Sheet2!A:A}<>"")

Suppose you have:

   A    B   C   D   E   F
1: 1    2   3   4   5   6

It's possible to concatenate slices as either rows or columns.

For additional columns (same row), use a comma. ={$A1:$C1,$D1:$F1} yields:

1   2   3   4   5   6

For additional rows (same columns), use a semicolon. ={$A1:$C1;$D1:$F1} yields:

1   2   3
4   5   6

At first when I tried ={Sheet1!A:A; Sheet2!A:A}, I thought it didn't work because I could only see results from the first sheet. Turned out it was including all the blank cells too!

To filter out blank and empty cells while preserving duplicates (unlike =UNIQUE) and without repeating yourself (unlike =FILTER()), you can use =QUERY(), like so:

=QUERY(
{March!A1:Z; April!A2:Z; May!A2:Z},
"select * where Col1 != '' and Col1 is not null",
0)

(Note that I am including the header row from the first sheet, and omitting it from the other sheets).

If your sheets don't contain cells with empty text, you can omit Col1 != '' and.