EIP1559 gas math in TS

getBaseFeeInNextBlock.ts

import { BigNumber } from 'ethers'

const BASE_FEE_MAX_CHANGE_DENOMINATOR = 8

export const getBaseFeeInNextBlock = (
  currentBaseFeePerGas: BigNumber,
  currentGasUsed: BigNumber,
  currentGasLimit: BigNumber
): BigNumber => {
  const currentGasTarget = currentGasLimit.div(2)

  if (currentGasUsed.eq(currentGasTarget)) {
    return currentBaseFeePerGas
  } else if (currentGasUsed.gt(currentGasTarget)) {
    const gasUsedDelta = currentGasUsed.sub(currentGasTarget)
    const baseFeePerGasDelta = currentBaseFeePerGas
      .mul(gasUsedDelta)
      .div(currentGasTarget)
      .div(BASE_FEE_MAX_CHANGE_DENOMINATOR)

    return currentBaseFeePerGas.add(baseFeePerGasDelta)
  } else {
    const gasUsedDelta = currentGasTarget.sub(currentGasUsed)
    const baseFeePerGasDelta = currentBaseFeePerGas
      .mul(gasUsedDelta)
      .div(currentGasTarget)
      .div(BASE_FEE_MAX_CHANGE_DENOMINATOR)

    return currentBaseFeePerGas.sub(baseFeePerGasDelta)
  }
}