prtm_calculate_protein_mass.js

const sequence = `HGMWTHHCWWMSEGNWPGPNTFAPMTRAVTDPCLKQTAKIMFAMYQMTRVEAPLLRNLGSLKLYMECSWENHSSPQGYAYADPHHPWGQFPIEDTKRYPFPNRNTKQAQLTKDMWPWLDEMDNKDVYIHRVDTWHKAWTMRMHHCQDMTSWRYHLCTYCKGSIAEKHIDITCGGDMHGWYGAQYDYAWWECRYWIEVRVKLLYHNFHTWHTTYYFVHYRITPDDEQYGGGGNDYKTFHTEWWCGVEGTDPFFKAIFEDICIMVIYCDKIIKHLSNFSSKPDEMNILPNDWAATRWAGATYDPMPDDDFAEVWNYANNIDKFKELELKQKGWWGQAILRTRTLPWIDFNQWCCIEIMVGCRSPNIWMSMCLNEAVNMDVQLMAIYNRDGYPESQHRYTQLHAYAGMAPAEAGTTWGPHIFCNDHTLEEHNTCWGRYCQVNWRPVSIWCRRWTNTAMMPEKCDMTWHDYVYTRIYWWFDVIPWNVCWMDEILKNKGYDIQRISLMMQSSYQTWCSNKPAREDRYCLWWNYWKLDSKHNYVQPGMYRCNAHYLTSKAMLYNWYSQCHVQALTNFYAWCFEVKEVMAWTCWRKNTHDSMIEHENITDCGPQFLRQPQAGNQYTYTWNEPIMHKWEQMEGDTCSKLMLCYRNQGQRNCYKIMKLQVQKYQYVFPREWHYRWHPLWHCCDVRTTCRGLSTGVWFEEQQNRCAHELWLDDHDINCNWSLFELISMKYEVFALLAMKQLEQHLSEHRVNKSEWLKQTGDVWSFCIPHGRWFHDQNNECCATKSHMFCMNRLLVAIQPPPCGNWGLCFGKNQRQDHHYCYVGLICERDEHDEAAMYKGGIVGHQQNNGMTRNCYGVTEEIVSKGELGMAIPHNVPMHGAYRNKKWSEHLMQKQNSGDVLPAHCVKIEHWLSQSNFVNATRKDPRFNPGPQAPDLGIQMVKKHQKDGI`;

const mmt = {
    'A'   : 71.03711,
    'C'   : 103.00919,
    'D'   : 115.02694,
    'E'   : 129.04259,
    'F'   : 147.06841,
    'G'   : 57.02146,
    'H'   : 137.05891,
    'I'   : 113.08406,
    'K'   : 128.09496,
    'L'   : 113.08406,
    'M'   : 131.04049,
    'N'   : 114.04293,
    'P'   : 97.05276,
    'Q'   : 128.05858,
    'R'   : 156.10111,
    'S'   : 87.03203,
    'T'   : 101.04768,
    'V'   : 99.06841,
    'W'   : 186.07931,
    'Y'   : 163.06333
};

let total = 0;
for (seq of sequence.split('')) {
    total += mmt[seq];
}

console.log(total);