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ES6 の for-of 文でプロパティの値と一緒に index の値も取得する

Last updated at Posted at 2019-02-09

ES6 の for-of 文を使うとき, index の値も同時に取得したい状況が稀によくある.

これは Object.entries() の分割代入を行うことで実現できる:

const numArray = [0, 1, 2, 42]

for (const [index, num] of numArray.entries()) {
  console.log(`index: ${index}, num: ${num}`)
}

// index: 0, num: 0
// index: 1, num: 1
// index: 2, num: 2
// index: 3, num: 42

TypeScript の場合

TypeScript で targetES5 以下にしているとき, デフォルトでは上記のコードはコンパイルエラーとなる (TypeScript 3.2 で確認):

TS2569: Type 'IterableIterator<[number, number]>' is not an array type or a string type. Use compiler option '--downlevelIteration' to allow iterating of iterators.

エラーメッセージにあるように, --downlevelIteration オプションをつければコンパイルが通るようになる1:

tsconfig.json
{
    "compilerOptions": {
        "downlevelIteration": true,
        "target": "es5"
    }
}

参考にした記事

  1. https://www.typescriptlang.org/docs/handbook/compiler-options.html

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