function rewardPerToken() public view returns (uint) {
        if (totalSupply == 0) {
            return rewardPerTokenStored;
        }
        return
            rewardPerTokenStored +
            (((block.timestamp - lastUpdateTime) * rewardRate * 1e18) / totalSupply);
    }

      describe("rewardPerToken", () => {
            it("Returns the reward amount of 1 share based off the locked up time", async () => {
                
                await mockWETH.connect(player1).approve(mutuals.address, amount0)
                await achiever.connect(player1).approve(mutuals.address, amount1)
                await mutuals.connect(player1).addLiquidity(amount0, amount1)
                // await moveTime(SECONDS_IN_A_DAY)
                // await moveBlocks(1)
                // let reward = await mutuals.rewardPerToken()
                // let expectedReward = "86"
                // assert.equal(reward.toString(), expectedReward)

                await moveTime(SECONDS_IN_A_YEAR)
                await moveBlocks(1)
                reward = await mutuals.rewardPerToken()
                expectedReward = "31536"
                console.log(ethers.utils.formatEther(reward))
                assert.equal(reward.toString(), expectedReward)
            })
        })

    rewardPerToken
Moving blocks...
Moved forward in time  seconds
Moving blocks...
Moved  blocks
0.000000000000031536
      ✓ Returns the reward amount of 1 share based off the locked up time

I was doing some unit testing and notice that reward.toString() would simply ignore the factor of 1e18. The display result in Remix would include 1e18 though.

Is that how Javacript interpret Big Number? Thanks.