Transaction is able to estimate fees

src/transaction.js

@@ -377,6 +377,15 @@ Transaction.prototype.validateSig = function(index, script, sig, pub) {
                                       convert.coerceToBytes(pub));
 }
 
+Transaction.feePerKb = 20000
+Transaction.prototype.estimateFee = function(feePerKb){
+  var feePerKb = feePerKb || Transaction.feePerKb
+  var size = this.ins.length * 180 + this.outs.length * 34 + 10
+  var sizeInKb = BigInteger.valueOf(Math.ceil(size / 1000))
+
+  return BigInteger.valueOf(feePerKb).multiply(sizeInKb).intValue()
+}
+
 var TransactionIn = function (data) {
     if (typeof data == "string")
         this.outpoint = { hash: data.split(':')[0], index: data.split(':')[1] }

test/transaction.js

@@ -2,6 +2,10 @@ var Transaction = require('../src/transaction').Transaction
 var convert = require('../src/convert')
 var ECKey = require('../src/eckey').ECKey
 var assert = require('assert')
+var fixtureTxes = require('./fixtures/mainnet_tx')
+var fixtureTx1Hex = fixtureTxes.prevTx
+var fixtureTx2Hex = fixtureTxes.tx
+var fixtureTxBigHex = fixtureTxes.bigTx
 
 describe('Transaction', function() {
   describe('deserialize', function() {
@@ -170,6 +174,22 @@ describe('Transaction', function() {
       })
     })
 
+    describe('estimateFee', function(){
+      it('works for fixture tx 1', function(){
+        var tx = Transaction.deserialize(fixtureTx1Hex)
+        assert.equal(tx.estimateFee(), 20000)
+      })
+
+      it('works for fixture big tx', function(){
+        var tx = Transaction.deserialize(fixtureTxBigHex)
+        assert.equal(tx.estimateFee(), 60000)
+      })
+
+      it('allow feePerKb to be passed in as an argument', function(){
+        var tx = Transaction.deserialize(fixtureTx2Hex)
+        assert.equal(tx.estimateFee(10000), 10000)
+      })
+    })
   })
 
 })