how to create Function Selector (Method IDs)

Question

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 ?

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.