Support for mapHeaders to filter columns and edit the printed headers

index.js

@@ -6,6 +6,7 @@ var CsvWriteStream = function(opts) {
   if (!opts) opts = {}
   stream.Transform.call(this, {objectMode:true, highWaterMark:16})
 
+  this.mapHeaders = opts.mapHeaders || function(header) { return header }
   this.sendHeaders = opts.sendHeaders !== false
   this.headers = opts.headers || null
   this.separator = opts.separator || opts.seperator || ','
@@ -51,21 +52,34 @@ CsvWriteStream.prototype._transform = function(row, enc, cb) {
   if (!isArray && !this.headers) this.headers = Object.keys(row)
 
   if (this._first && this.headers) {
+
+    var printedHeaders = [];
+
+    for (var i = 0; i < this.headers.length; i++) {
+      var key = this.headers[i]
+      var printedHeader = this.mapHeaders(key, i)
+      printedHeaders.push(printedHeader)
+      if (typeof printedHeader !== 'string') {
+        this.headers[i] = null
+      }
+    }
+
     this._first = false
 
     var objProps = []
     var arrProps = []
     var heads = []
 
     for (var i = 0; i < this.headers.length; i++) {
-      arrProps.push('obj['+i+']')
-      objProps.push(gen('obj', this.headers[i]))
+      if (this.headers[i] !== null) {
+        arrProps.push('obj['+i+']')
+        objProps.push(gen('obj', this.headers[i]))
+      }
     }
 
     this._objRow = this._compile(objProps)
     this._arrRow = this._compile(arrProps)
-
-    if (this.sendHeaders) this.push(this._arrRow(this.headers))
+    if (this.sendHeaders) this.push(this._arrRow(printedHeaders || this.headers))
   }
 
   if (isArray) {

test.js

@@ -86,6 +86,56 @@ test('encode from object w/ auto headers', function(t) {
   writer.end()
 })
 
+test('encode from object w/ auto headers and one header skipped', function(t) {
+  var writer = csv({
+    mapHeaders: function (header) {
+      return header !== 'foo' ? header : null
+    }
+  })
+
+  writer.pipe(concat(function(data) {
+    t.equal(data.toString(), 'hello,baz\nworld,taco\n')
+    t.end()
+  }))
+
+  writer.write({hello: "world", foo: "bar", baz: "taco"})
+  writer.end()
+})
+
+test('encode from object w/ auto headers and change the header', function(t) {
+  var writer = csv({
+    mapHeaders: function (header) {
+      return header !== 'foo' ? header : 'Foo'
+    }
+  })
+
+  writer.pipe(concat(function(data) {
+    t.equal(data.toString(), 'hello,Foo,baz\nworld,bar,taco\n')
+    t.end()
+  }))
+
+  writer.write({hello: "world", foo: "bar", baz: "taco"})
+  writer.end()
+})
+
+test('encode from object edit header and filter one column', function(t) {
+  var writer = csv({
+    headers: ['hello', 'foo', 'baz'],
+    mapHeaders: function (header) {
+      return header === 'foo' ? null
+          : header.substring(0,1).toUpperCase() + header.substring(1)
+    }
+  })
+
+  writer.pipe(concat(function(data) {
+    t.equal(data.toString(), 'Hello,Baz\nworld,taco\n')
+    t.end()
+  }))
+
+  writer.write(["world", "bar", "taco"])
+  writer.end()
+})
+
 test('no headers specified', function(t) {
   var writer = csv()