Refactored chunkIndex into isFirstChunk

papaparse.js

@@ -379,7 +379,7 @@
 		this._rowCount = 0;
 		this._start = 0;
 		this._nextChunk = null;
-		this._chunkIndex = 0;
+		this.isFirstChunk = true;
 		this._completeResults = {
 			data: [],
 			errors: [],
@@ -389,13 +389,14 @@
 
 		this.parseChunk = function(chunk)
 		{
-			if (this._chunkIndex == 0 && isFunction(this._config.beforeFirstChunk)) {
-				var result = this._config.beforeFirstChunk(chunk);
-				if (result !== undefined)
-					chunk = result;
+			// First chunk pre-processing
+			if (this.isFirstChunk && isFunction(this._config.beforeFirstChunk))
+			{
+				var modifiedChunk = this._config.beforeFirstChunk(chunk);
+				if (modifiedChunk !== undefined)
+					chunk = modifiedChunk;
 			}
-
-			this._chunkIndex++;
+			this.isFirstChunk = false;
 
 			// Rejoin the line we likely just split in two by chunking the file
 			var aggregate = this._partialLine + chunk;

player/player.js

@@ -104,7 +104,8 @@ function buildConfig()
 		download: $('#download').prop('checked'),
 		fastMode: $('#fastmode').prop('checked'),
 		skipEmptyLines: $('#skipEmptyLines').prop('checked'),
-		chunk: $('#chunk').prop('checked') ? chunkFn : undefined
+		chunk: $('#chunk').prop('checked') ? chunkFn : undefined,
+		beforeFirstChunk: undefined,
 	};
 
 	function getLineEnding()

tests/test-cases.js

@@ -1322,7 +1322,7 @@ var CUSTOM_TESTS = [
 		}
 	},
 	{
-		description: "beforeFirstChunk can manipulate first chunk",
+		description: "beforeFirstChunk manipulates only first chunk",
 		expected: 7,
 		run: function(callback) {
 			var updates = 0;
@@ -1340,6 +1340,25 @@ var CUSTOM_TESTS = [
 				}
 			});
 		}
+	},
+	{
+		description: "First chunk not modified if beforeFirstChunk returns nothing",
+		expected: 8,
+		run: function(callback) {
+			var updates = 0;
+			Papa.parse("/tests/long-sample.csv", {
+				download: true,
+				chunkSize: 500,
+				beforeFirstChunk: function(chunk) {
+				},
+				step: function(response) {
+					updates++;
+				},
+				complete: function() {
+					callback(updates);
+				}
+			});
+		}
 	}
 
 ];