Implement https://github.com/mholt/PapaParse/issues/612

docs/docs.html

@@ -510,8 +510,19 @@ var csv = Papa.unparse({
 									<code>transformHeader</code>
 								</td>
 								<td>
-									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument and the index as second.<br>
-									Only available starting with version 5.0.
+									A function to apply on each header. Requires <code>header</code> to be <code>true</code>. The function receives the header as its first argument and the index (the column number) as second. When used in conjunction with config.headerLines, the function receives a third and fourth parameter which are:
+									<ul>
+										<li>the existing header for that column based on previous lines iteration ('' if no previous lines)</li>
+										<li>the rowNumber (0 for the first line)</li>
+									</ul>
+								</td>
+							</tr>
+							<tr>
+								<td>
+									<code>headerLines</code>
+								</td>
+								<td>
+									The number of rows which will be used to transform into a header, and removed from the rest of the data. Requires header to be true. Default 1.
 								</td>
 							</tr>
 							<tr>

papaparse.js

@@ -1180,24 +1180,26 @@ License: MIT
 			if (!_results)
 				return;
 
-			function addHeader(header, i)
+			var headerLines = _config.headerLines || 1;
+			function addHeader(j, header, i)
 			{
 				if (isFunction(_config.transformHeader))
-					header = _config.transformHeader(header, i);
+					header = _config.transformHeader(header, i, _fields[i] || '', j);
 
-				_fields.push(header);
+				_fields[i] = header;
 			}
-
 			if (Array.isArray(_results.data[0]))
 			{
-				for (var i = 0; needsHeaderRow() && i < _results.data.length; i++)
-					_results.data[i].forEach(addHeader);
-
-				_results.data.splice(0, 1);
+				for (var j = 0; j < Math.min(headerLines, _results.data.length); j++)
+				{
+					// A function which takes two arguments (header, i) where j is set by the fact it is called in this loop. It then calls addHeader() with all three arguments
+					_results.data[j].forEach(addHeader.bind(null, j));
+				}
+				_results.data.splice(0, headerLines);
 			}
 			// if _results.data[0] is not an array, we are in a step where _results.data is the row.
 			else
-				_results.data.forEach(addHeader);
+				_results.data.forEach(addHeader.bind(null, 0));
 		}
 
 		function shouldApplyDynamicTyping(field) {

tests/test-cases.js

@@ -767,6 +767,15 @@ var PARSE_TESTS = [
 			errors: []
 		}
 	},
+	{
+		description: "Header row is generated from multiple rows when transformheader function is provided and config.headerLines is set",
+		input: "A,B,C\r\nD,E,F\r\nG,H,I\r\nadg,beh,cfi",
+		config: { header: true, transformHeader: function(header, i, acc, j) { return acc + (j % 2 ? header.toLowerCase() : header); }, headerLines: 3 },
+		expected: {
+			data: [{"AdG": "adg", "BeH": "beh", "CfI": "cfi"}],
+			errors: []
+		}
+	},
 	{
 		description: "transformHeader accepts and optional index attribute",
 		input: 'A,B,C\r\na,b,c',