decodeTransaction.html

<script src="web3.min.js"></script>
<script>
  const web3 = new Web3();
  decodeTransaction = () => {
    document.querySelector('#result').innerText = '';
    const abi = JSON.parse(document.querySelector('#abi').value);
    const functionsAbi = abi.filter(item => item.type === 'function').map(item => {
      item.signature = web3.eth.abi.encodeFunctionSignature(item);
      return item;
    });
    const input = document.querySelector('#input').value;
    const functionAbi = functionsAbi.find(item => input.indexOf(item.signature) === 0);
    if (functionAbi) {
      const functionName = functionAbi.name;
      let functionArgs;
      try {
        functionArgs = web3.eth.abi.decodeParameters(functionAbi.inputs, input.slice(10));
        delete functionArgs.__length__;
      } catch (e) {
        console.error(e);
      }
      document.querySelector('#result').innerText = JSON.stringify({
        functionName: functionName,
        functionArgs: functionArgs,
      }, null, 4);
    }
  }
</script>
<style>
  textarea {
    width: 100%;
    height: 100px;
  }
</style>
<h1>Decode Ethereum Transaction Online</h1>
ABI <textarea id="abi" placeholder='[{"inputs": [{"name": "_to", "type": "address"}, {"name": "_amount", "type": "uint256"}], "name": "transfer", "type": "function"}]'></textarea>
<br>
Input <textarea id="input" placeholder="0xa9059cbb0000000000000000abcdef0123456789abcdef0123456789abcdef011"></textarea>
<br>
<button onclick="decodeTransaction()">Decode</button>
<pre id="result"></pre>
* web3 can't decode strings, use bytes32 instead