Allow custom function to map columns

Readme.md

@@ -72,6 +72,8 @@ The options are also passed to the underlying transform stream, so you can pass
   objectMode: false,
 
   // if set to true, uses first row as keys -> [ { column1: value1, column2: value2 }, ...]
+  // a function can be passed to columns to map the array of columns
+  // columns: (cols) => cols.map(col => col.toLowerCase())
   columns: false
 }
 ```

csv-streamify.js

@@ -11,7 +11,7 @@ var through = require('through2')
  * @param {string} [opts.quote='"'] The quote character
  * @param {string} [opts.empty=''] Which character to return for empty fields
  * @param {boolean} [opts.objectMode=true] Whether to return an object or a buffer per line
- * @param {boolean} [opts.columns=false] Whether to parse headers
+ * @param {boolean|function} [opts.columns=false] Whether to parse headers or a function to parse headers
  * @param {function} [cb] Callback function
  */
 module.exports = function (opts, cb) {
@@ -53,7 +53,11 @@ function createParser (opts, state) {
 
     if (opts.hasColumns) {
       if (state.lineNo === 0) {
-        state._columns = state._line
+        if (typeof opts.hasColumns === 'function') {
+          state._columns = opts.hasColumns(state._line)
+        } else {
+          state._columns = state._line
+        }
         state.lineNo += 1
         reset()
         return

test/csv-streamify.js

@@ -157,6 +157,19 @@ describe('object mode', function () {
 
     str('COL0,COL1\ncol0,col1\ncol2,col3').pipe(parser)
   })
+
+  it('should emit multiple columns parsed by custom function', function (done) {
+    var parser = csv({
+      columns: (cols) => cols.map(col => col.toLowerCase())
+    }, function (err, res) {
+      if (err) return done(err)
+      assert.deepEqual(res[0], { col0: 'col0', col1: 'col1' })
+      assert.deepEqual(res[1], { col0: 'col2', col1: 'col3' })
+      done()
+    })
+
+    str('COL0,COL1\ncol0,col1\ncol2,col3').pipe(parser)
+  })
 })
 
 describe('edge cases', function () {