Get amount from Lightning invoice string

getLightningInvoiceAmount.js

import _ from 'lodash';

const getInvoiceAmount = (invoice) => {
    const cleanInvoice = invoice.toLowerCase();

    let removedNetwork;
    if (_.startsWith(cleanInvoice, 'lnbc')) {
        removedNetwork = cleanInvoice.replace('lnbc', '');
    } else if (_.startsWith(cleanInvoice, 'lntb')) {
        removedNetwork = cleanInvoice.replace('lntb', '');
    } else if (_.startsWith(cleanInvoice, 'lnbcrt')) {
        removedNetwork = cleanInvoice.replace('lnbcrt', '');
    } else {
        throw 'Not a BOLT-11 invoice';
    }

    const multiplier = removedNetwork.match(/[a-zA-Z]/).pop();

    let invoiceAmount;
    if (multiplier === 'n') {
        const multiplierPosition = removedNetwork.indexOf('n');
        invoiceAmount = removedNetwork.substring(0, multiplierPosition - 1);
    } else if (multiplier === 'u') {
        const multiplierPosition = removedNetwork.indexOf('u');
        invoiceAmount = removedNetwork.substring(0, multiplierPosition);
        invoiceAmount += '00';
    } else if (multiplier === 'm') {
        const multiplierPosition = removedNetwork.indexOf('m');
        invoiceAmount = removedNetwork.substring(0, multiplierPosition);
        invoiceAmount += '00000';
    } else {
        throw 'Something went wrong with BOLT-11 decoding';
    }

    const amountAsNumber = Number(invoiceAmount);
    return amountAsNumber;
};

export default getInvoiceAmount;