Lesson 9, Getting an error in unit testing, TypeError: interval.toNumber is not a function

[KartikC137]

Script:

const { assert, expect } = require("chai")
const { network, getNamedAccounts, deployments, ethers } = require("hardhat")
const {
    developmentChains,
    networkConfig,
} = require("../../helper-hardhat-config")

!developmentChains.includes(network.name)
    ? describe.skip
    : describe("Raffle", async function () {
          let raffle,
              vrfCoordinatorV2_5Mock,
              raffleEntranceFee,
              deployer,
              interval
          const chainId = network.config.chainId

          beforeEach(async function () {
              deployer = (await getNamedAccounts()).deployer
              await deployments.fixture(["all"])
              raffle = await ethers.getContract("Raffle", deployer)
              vrfCoordinatorV2_5Mock = await ethers.getContract(
                  "VRFCoordinatorV2_5Mock",
                  deployer,
              )
              raffleEntranceFee = await raffle.getEntranceFee()
              interval = await raffle.getInterval()
          })

          describe("constructor", async function () {
              it("intializes the raffle correctly", async function () {
                  // Ideally we make out tests have just 1 assert per "it"

                  const raffleState = await raffle.getRaffleState()
                  assert.equal(raffleState.toString(), "0")
                  assert.equal(
                      interval.toString(),
                      networkConfig[chainId]["interval"],
                  )
              })
          })

          describe("enterRaffle", async function () {
              it("reverts when you don't pay enough", async function () {
                  await expect(
                      raffle.enterRaffle(),
                  ).to.be.revertedWithCustomError(
                      raffle,
                      "Raffle__NotEnoughETHEntered",
                  )
              })
              it("records players when they enter", async function () {
                  //raffle Entrance Fee
                  await raffle.enterRaffle({ value: raffleEntranceFee })
                  const playerFromContract = await raffle.getPlayer(0)
                  assert.equal(playerFromContract, deployer)
              })

              it("emits event on enter", async function () {
                  await expect(
                      raffle.enterRaffle({ value: raffleEntranceFee }),
                  ).to.emit(raffle, "RaffleEnter")
              })
              it("doesn't allow entrance when raffle is calculating", async function () {
                  await raffle.enterRaffle({ value: raffleEntranceFee })
                  await network.provider.send("evm_increaseTime", [
                      interval.toNumber() + 1,
                  ])
                  await network.provider.send("evm_mine", [])

                  // Pretend to be a Chainklink Keeper

                  await raffle.performUpkeep([])
                  await expect(
                      raffle.enterRaffle({ value: raffleEntranceFee }),
                  ).to.be.revertedWithCustomError(raffle, "Raffle__NotOpen")
              })
          })
      })

However interval.toString() works

Link to my repo : https://github.com/KartikC137/Learning-Web3-FCC-/tree/testing