How to use the decimal.js.Decimal function in decimal

private onEdit(index: number) {
		const t = (this.props.transactions.get(index) as Transaction);
		this.props.navigation.navigate("Modify", {
			editTransaction: {
				index,
				transaction: t,
			},
			isExpense: new Decimal(t.amount).isNegative(),
		});
	}

function createDailyTransaction(
	day: moment.Moment,
	incomeType: string,
	income: Decimal): Transaction {
	return {
		amount: incomeType === "daily" ? income : new Decimal(income).div(new Decimal(day.daysInMonth())).toDecimalPlaces(2),
		comment: "AUTOMATIC",
		date: day.toDate().getTime(),
	};
}

static fromCoins = (amount, currency) => {
        currency = currency || {};
        if (currency.precision === undefined)
            throw new Error('A valid currency must be provided');

        if (amount instanceof BigNumber)
            amount = amount.toString();

        amount = new Decimal(amount);
        amount = amount.div(Math.pow(10, currency.precision));

        return new Money(amount, currency);
    };

const normalizePrice = (price, pair) => {
    if (price instanceof BigNumber)
        price = price.toString();

    return new Decimal(price)
        .div(MATCHER_SCALE)
        .div(Math.pow(10, pair.priceAsset.precision - pair.amountAsset.precision));
};

onClick={e => {
              let value = new Decimal(valueTrue).plus(step).valueOf()
              if (isAddDisabled) {
                return
              }
              if (step > 0) {
                if (this.willReachMax) {
                  value = max
                }
              } else {
                if (this.willReachMin) {
                  value = min
                }
              }
              this.update(value)
            }}
          >

onClick={e => {
              let value = new Decimal(this.getInputNumber()).minus(step).valueOf()
              if (isMinusDisabled) {
                return
              }
              if (step > 0) {
                if (this.willReachMin) {
                  value = min
                }
              } else {
                if (this.willReachMax) {
                  value = max
                }
              }
              this.update(value)
            }}
          >

export const numberToString = (val1: string, val2: number): string => {
  const amount = val1 ? val1 : '0'
  const decimals = new Decimal(val2 ? val2 : 0)
  const bigAmount = new Decimal(amount)
  const bigMultiplier = new Decimal(10).pow(decimals)
  return  bigAmount.div(bigMultiplier).toFixed()
}

constructor(amount, currency) {
        if (amount === undefined)
            throw new Error('Amount is required');

        if (currency === undefined)
            throw new Error('Currency is required');

        if (amount instanceof BigNumber)
            amount = amount.toString();

        this.amount = new Decimal(amount)
            .toDecimalPlaces(currency.precision, Decimal.ROUND_FLOOR);
        this.currency = currency;
    }

totalTokens() {
    const totalStaked = this.totalStakedToSelf().plus(this.totalStakedToOthers());
    const totalBeingUnstaked = this.totalBeingUnstaked();
    const totalTokens = this.tokens()[this.chainSymbol] || new Decimal(0);

    return totalStaked.plus(totalBeingUnstaked).plus(totalTokens);
  }