As stated in ABI specs that 0xcdcd77c0: the Method ID. This is derived as the first 4 bytes of the Keccak hash of the ASCII form of the signature baz(uint32,bool).

So, I have to create method IDs (function selectors) of my contract functions through javascript/nodejs/web3js. Suppose i have an array of functions name and their parameters (extracted from ABI JSON file)- i.e. funcNameAndParams[]. One of its element is as follow;

  contractNo: 0,
  funcName: 'getSupplyRate',
  params: [ 'address', 'uint256', 'uint256' ]
  1. Is it possible to generate Method IDs, having above information ? if yes, how could i proceed ? I have read ABI specs but could not found how to code it in JavaScript/Nodejs/web3.js
  2. If I have contract source code as well, then could i proceed ? Means generating Methods IDs from json ABIs will be easy or from .sol file ?

1 Answer 1


With web3.js v1.2.x, you can use function web3.eth.abi.encodeFunctionSignature:

const selector = web3.eth.abi.encodeFunctionSignature({
    type: "function",
    name: yourObj.funcName,
    inputs: yourObj.params.map(param => ({type: param}))

Or simply:

const selector = web3.eth.abi.encodeFunctionSignature("getSupplyRate(address,uint256,uint256)");

In the second example, note that you should avoid any spaces between the parameter types.

  • thanks a lot... works for me...
    – Amir Ali
    Commented Dec 10, 2019 at 12:40
  • @AmirAli: NP :) Commented Dec 10, 2019 at 12:44

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