Metamask style balance formatter

format-balance.ts

import { BigNumber } from "@ethersproject/bignumber";
import { formatUnits, parseUnits } from "@ethersproject/units";

/**
 * @param balance Token balance in BigNumber format
 * @param decimals Token decimals
 * @param fixed The number of decimal places to show
 * @returns string representation of the balance
 */
export function bigNumberToTrimmed(
  balance: BigNumber,
  decimals = 18,
  fixed = 4
): string {
  if (!(balance instanceof BigNumber)) {
    throw new Error(
      `"Invalid balance type: ${typeof balance}. Should be BigNumber.`
    );
  }

  if (balance.isZero()) {
    return "0";
  } else if (
    BigNumber.from(parseUnits(formatUnits("1", fixed), decimals)).gt(balance)
  ) {
    return `< ${formatUnits("1", fixed)}`;
  } else {
    const [l, r] = formatUnits(balance.toString()).split(".");

    return Intl.NumberFormat("en-US", {
      minimumFractionDigits: 0,
      maximumFractionDigits: fixed,
    })
      .format(
        parseFloat(`${l}.${r?.slice(0, fixed + 1).replace(/0+$/, "") ?? 0}`)
      )
      .toString();
  }
}

format.balance.spec.ts

import { BigNumber } from "@ethersproject/bignumber";
import { describe, expect, it } from "vitest";
import { parseUnits, formatUnits } from "ethers/lib/utils";

import { bigNumberToTrimmed, hexToNumber, numberToHex } from "./format-balance";

describe("bigNumberToTrimmed", () => {
  it("test cases", () => {
    expect(bigNumberToTrimmed(BigNumber.from("0"))).toEqual("0");
    expect(bigNumberToTrimmed(BigNumber.from("1"))).toEqual("< 0.0001");
    expect(bigNumberToTrimmed(BigNumber.from("100000000000000"))).toEqual("0.0001");
    expect(bigNumberToTrimmed(BigNumber.from("1000000000000000000"))).toEqual("1");
    expect(bigNumberToTrimmed(BigNumber.from("1000000000000000000000"))).toEqual("1,000");
    expect(bigNumberToTrimmed(BigNumber.from(parseUnits(formatUnits("123456789", 4), 18)))).toEqual("12,345.6789");
  });
});