JavaScriptでリクエストするときCORSエラーが出てハマったので解決策をメモしておく。
ブツ
情報源
ハマったコード抜粋
const address = `MEHCqJbgiNERCH3bRAtNSSD9uxPViEX1nu`
const url = `https://blockbook.electrum-mona.org/api/v2/address/${address}`
const data = {
method: 'GET',
headers: {
'Accept': 'application/json',
'Content-Type': 'application/json',
},
}
const res = await fetch(url, data)
const json = await res.json()
実行すると以下のようなエラーが出た。
Access to fetch at 'https://blockbook.electrum-mona.org/api/v2/address/MEHCqJbgiNERCH3bRAtNSSD9uxPViEX1nu' from origin 'https://localhost' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.
キーワードからみるに、またCORSエラーらしい。ウザすぎる。content-typeがどうとか言ってる。これはHTTPヘッダでレスポンスのMIMEタイプを指定するプロパティ。上記コードでは'Content-Type': 'application/json',と指定しているが、not allowedだという。ならどうすりゃいいのさ？ ググると以下発見。
どうやらContent-Typeの値を以下のうちのいずれかにしないとダメらしい。
application/x-www-form-urlencodedmultipart/form-datatext/plain
よくわからない。このAPI、JSONを返すものでは？ なのに、なぜapplication/jsonを禁ずるのか。たぶんセキュリティ的に何かあるのだろう。
まあいいや。とりあえずJSONもただのテキストだし、text/plainで取得したあとその文字列をJSON化すればイケるだろう。
解決コード
const address = `MEHCqJbgiNERCH3bRAtNSSD9uxPViEX1nu`
const url = `https://blockbook.electrum-mona.org/api/v2/address/${address}`
const data = {
method: 'GET',
headers: {
'Accept': 'application/json',
'Content-Type': 'text/plain', // ここを変えただけ！
},
}
const res = await fetch(url, data)
const json = await res.json()
{
"page": 1,
"totalPages": 1,
"itemsOnPage": 1000,
"address": "MEHCqJbgiNERCH3bRAtNSSD9uxPViEX1nu",
"balance": "1682705730",
"totalReceived": "2866800590",
"totalSent": "1184094860",
"unconfirmedBalance": "0",
"unconfirmedTxs": 0,
"txs": 149,
"txids": [
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"97709db041c0a6a0ad18b9118ac37b0cbc1b09df89ed074dac3e3c63a5d195f7",
"1623d729a22e5a7308351aefd19a642265c056cc210de2eae9c0b979627e42ea",
"fff7f83ac53af5cef66fd5d052ffd1731977001f1bc08687f073d2e1b959a9bc",
"727b6c03bd71e0ee51c73cc236f7829a6591ed23ed0097e0a0ca385d82a30714",
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]
}
取引データを取得できた！ 執筆時点でtxidsは149件あった。
"itemsOnPage": 1000,は一回のリクエストで取得できる上限値だと思われる。それ以上を取得するにはページネーション処理が必要っぽい。
あと、ソートは新しい順らしい。search_raw_transactionsのときは古い順だったのに逆だね。
展望
妙なハマりポイントがあったが何とか成功。たぶん他のAPIも同じようにイケるはず。
あとはこうして入手したデータをローカルに保存できれば、技術的な課題はクリアできそう。
そしたらトランザクション情報から支払か受取かを判別する方法がわからないせいで生じた取引集計とユーザ相関図のバグを治せるはず。
やっと光明が見えてきた。