Create OP_RETURN tx with Bitcoinjs-lib I know how to create a basic TX:

var bitcoin = require('bitcoinjs-lib')

var tx = new bitcoin.Transaction()

// Add the input (who is paying) of the form [previous transaction hash, index of the output to use]
tx.addInput("aa94ab02c182214f090e99a0d57021caffd0f195a81c24602b1028b130b63e31", 0)

// Add the output (who to pay to) of the form [payee's address, amount in satoshis]
tx.addOutput("THEADDRESS", 15000)

// Initialize a private key using WIF
key = bitcoin.ECKey.fromWIF("L1uyy5qTuGrVXrmrsvHWHgVzW9kKdrp27wBC7Vs6nZDTF2BRUVwy")

// Sign the first input with the new key
tx.sign(0, key)

// Print transaction serialized as hex
console.log(tx.toHex())
// =&gt; 0100000001313eb630b128102b60241ca895f1d0ffca2170d5a0990e094f2182c102ab94aa000000008a47304402200169f1f844936dc60df54e812345f5dd3e6681fea52e33c25154ad9cc23a330402204381ed8e73d74a95b15f312f33d5a0072c7a12dd6c3294df6e8efbe4aff27426014104e75628573696aed32d7656fb35e9c71ea08eb6492837e13d2662b9a36821d0fff992692fd14d74fdec20fae29128ba12653249cbeef521fc5eba84dde0689f27ffffffff01983a0000000000001976a914ad618cf4333b3b248f9744e8e81db2964d0ae39788ac00000000

// You could now push the transaction onto the Bitcoin network manually (see https://blockchain.info/pushtx)


But I didn't find out how I could create a OP_RETURN transaction with bitcoinjs-lib in Node.js. Does anyone know?
Here:

var bitcoin = require('bitcoinjs-lib')

data = new Buffer("Melons.")

var tx = new bitcoin.TransactionBuilder()
tx.addInput("aa94ab02c182214f090e99a0d57021caffd0f195a81c24602b1028b130b63e31", 0)
tx.addOutput("some address", 15000)
ret = bitcoin.script.compile(
  [
    bitcoin.opcodes.OP_RETURN,
    data
  ])
tx.addOutput(ret, 0)
key = bitcoin.ECPair.fromWIF("L1uyy5qTuGrVXrmrsvHWHgVzW9kKdrp27wBC7Vs6nZDTF2BRUVwy")
tx.sign(0, key)
console.log(tx.build().toHex())


That should do it.

Also, if you have more than 40 bytes of data, your transaction will be non-standard.