TextDecoder を使って無理矢理 Windows-31J の Encoder を作る

encodeWindows31J.js

const encodeWindows31J = text => {
    const range = (from, to) => [...Array(to - from + 1).keys()].map(b => b + from)

    const islead = b => (b >= 0x81 && b <= 0x9F) || (b >= 0xe0 && b <= 0xef)
    const follows = [...range(0x40, 0x7e), ...range(0x80, 0xfc)]

    const decoder = new TextDecoder('Windows-31J')
    const sbmap = b => ({ [decoder.decode(new Uint8Array([b]))]: [b] })
    const dbmap = l => follows.reduce((mapping, f) => ({ ...mapping, [decoder.decode(new Uint8Array([l, f]))]: [l, f] }), {})

    const mapping = [...Array(256).keys()].reduce((mapping, b) => ({ ...mapping, ...islead(b) ? dbmap(b) : sbmap(b) }), {})

    return text.split('').reduce((bytes, c) => mapping[c] ? [...bytes, ...mapping[c]] : bytes, [])
}