fs: remove unnecessary argument check

Writable already assures that only Buffer's are passed to _write. Also
this is not the "correct" way to handle errors inside _write.

PR-URL: #29043
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/internal/fs/streams.js

@@ -3,7 +3,6 @@
 const { Math, Object } = primordials;
 
 const {
-  ERR_INVALID_ARG_TYPE,
   ERR_OUT_OF_RANGE
 } = require('internal/errors').codes;
 const { validateNumber } = require('internal/validators');
@@ -235,6 +234,9 @@ function WriteStream(path, options) {
 
   options = copyObject(getOptions(options, {}));
 
+  // Only buffers are supported.
+  options.decodeStrings = true;
+
   // For backwards compat do not emit close on destroy.
   if (options.emitClose === undefined) {
     options.emitClose = false;
@@ -295,11 +297,6 @@ WriteStream.prototype.open = function() {
 
 
 WriteStream.prototype._write = function(data, encoding, cb) {
-  if (!(data instanceof Buffer)) {
-    const err = new ERR_INVALID_ARG_TYPE('data', 'Buffer', data);
-    return this.emit('error', err);
-  }
-
   if (typeof this.fd !== 'number') {
     return this.once('open', function() {
       this._write(data, encoding, cb);

test/parallel/test-fs-write-stream.js

@@ -55,12 +55,13 @@ tmpdir.refresh();
 // Throws if data is not of type Buffer.
 {
   const stream = fs.createWriteStream(file);
-  common.expectsError(() => {
-    stream._write(42, null, function() {});
-  }, {
+  stream.on('error', common.expectsError({
     code: 'ERR_INVALID_ARG_TYPE',
-    type: TypeError,
-    message: 'The "data" argument must be of type Buffer. Received type number'
-  });
+    type: TypeError
+  }));
+  stream.write(42, null, common.expectsError({
+    code: 'ERR_INVALID_ARG_TYPE',
+    type: TypeError
+  }));
   stream.destroy();
 }