{"id":45295,"date":"2023-12-10T11:03:18","date_gmt":"2023-12-10T02:03:18","guid":{"rendered":"https:\/\/kinsta.com\/jp\/?p=45295&post_type=knowledgebase&preview_id=45295"},"modified":"2025-10-02T04:45:36","modified_gmt":"2025-10-01T19:45:36","slug":"typeerror-cannot-read-property-map-of-undefined","status":"publish","type":"post","link":"https:\/\/kinsta.com\/jp\/blog\/typeerror-cannot-read-property-map-of-undefined\/","title":{"rendered":"React\u306e\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u306e\u89e3\u6c7a\u65b9\u6cd5"},"content":{"rendered":"

\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\uff08\u672a\u5b9a\u7fa9\u306e\u30d7\u30ed\u30d1\u30c6\u30a3’map’\u3092\u8aad\u307f\u53d6\u308b\u3053\u3068\u304c\u3067\u304d\u307e\u305b\u3093\uff09\u306f\u3001React\u30a2\u30d7\u30ea\u3067\u767a\u751f\u3059\u308b\u53ef\u80fd\u6027\u306e\u3042\u308b\u30a8\u30e9\u30fc\u30e1\u30c3\u30bb\u30fc\u30b8\u3067\u3059\u3002\u3084\u3084\u30c7\u30d0\u30c3\u30b0\u304c\u8907\u96d1\u306b\u306a\u308b\u53ef\u80fd\u6027\u3082\u3042\u308a\u307e\u3059\u304c\u3001\u3053\u306e\u8a18\u4e8b\u304c\u304a\u5f79\u306b\u7acb\u3066\u308b\u306f\u305a\u3067\u3059\u3002<\/p>\n

\u4ee5\u4e0b\u3001\u4e3b\u306a\u539f\u56e0\u3068\u89e3\u6c7a\u7b56\u3092\u8a73\u3057\u304f\u3054\u7d39\u4ecb\u3057\u3066\u3044\u304d\u307e\u3059\u3002\u7d4c\u9a13\u8c4a\u5bcc\u306aReact\u958b\u767a\u8005<\/a>\u3067\u3042\u3063\u3066\u3082\u3001React\u3092\u3053\u308c\u304b\u3089\u4f7f\u3044\u59cb\u3081\u308b\u4eba\u3067\u3042\u3063\u3066\u3082\u3001\u30a8\u30e9\u30fc\u3092\u52b9\u7387\u7684\u306b\u89e3\u6d88\u3057\u3066\u3001\u30a2\u30d7\u30ea\u958b\u767a\u3092\u8ecc\u9053\u306b\u4e57\u305b\u307e\u3057\u3087\u3046\u3002
\n

<\/div><\/kinsta-auto-toc><\/p>\n

\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u306e\u539f\u56e0<\/h2>\n

\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u306f\u901a\u5e38\u3001React\u306e\u30b3\u30fc\u30c9\u5185\u306b\u672a\u5b9a\u7fa9\u306e\u5024\u306e\u30d7\u30ed\u30d1\u30c6\u30a3\u3084\u95a2\u6570\u306b\u30a2\u30af\u30bb\u30b9\u3057\u3088\u3046\u3068\u3059\u308b\u3068\u767a\u751f\u3057\u307e\u3059\u3002<\/p>\n

\u7c21\u5358\u306b\u8a00\u3048\u3070\u3001\u521d\u671f\u5316\u3055\u308c\u3066\u3044\u306a\u3044\u914d\u5217\u3084\u30c7\u30fc\u30bf\u3092\u53d7\u3051\u53d6\u3063\u3066\u3044\u306a\u3044\u914d\u5217\u306a\u3069\u3001\u672a\u5b9a\u7fa9\u306e\u5024\u3092\u30de\u30c3\u30d4\u30f3\u30b0\u3057\u3088\u3046\u3068\u3059\u308b\u3068\u30a8\u30e9\u30fc\u304c\u5f15\u304d\u8d77\u3053\u3055\u308c\u307e\u3059\u3002<\/p>\n

\u4f8b\u3048\u3070\u3001\u4ee5\u4e0b\u306e\u30b3\u30fc\u30c9\u3067\u306f\u3001JSON Placeholder<\/a>\u30c7\u30fc\u30bf\u304b\u3089Todo\u9805\u76ee\u3092\u53d6\u5f97\u3057\u3066\u3044\u307e\u3059\u304c\u3001API<\/a>\u30ea\u30af\u30a8\u30b9\u30c8\u304b\u3089\u306e\u30c7\u30fc\u30bf\u304c\u5230\u7740\u3059\u308b\u524d\u306bmap\u95a2\u6570\u304c\u547c\u3073\u51fa\u3055\u308c\u3066\u3044\u307e\u3059\u3002<\/p>\n

import { useState, useEffect } from 'react';\n\nfunction App() {\n  const [todos, setTodos] = useState();\n\n  useEffect(() => {\n    const getTodos = async () => {\n      const response = await fetch(\n        'https:\/\/jsonplaceholder.typicode.com\/todos?_limit=5'\n      );\n      const data = await response.json();\n      setTodos(data);\n    };\n    getTodos();\n  }, []);\n\n  console.log(todos);\n\n  return (\n    <div>\n      {todos.map((todo) => (\n        <div key={todo.id}>\n          <h2>Item: {todo.title}<\/h2>\n        <\/div>\n      ))}\n    <\/div>\n  );\n}\n\nexport default App;<\/code><\/pre>\n
\u3053\u3046\u306a\u308b\u3068\u3001\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u304c\u767a\u751f\u3057\u307e\u3059\u3002<\/p>\n
\"\u300cTypeError:
\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc<\/figcaption><\/figure>\n
\u914d\u5217\u306b\u30c7\u30fc\u30bf\u304c\u5165\u308b\u524d\u304b\u3089React<\/a>\u306bTodo\u306estate\u5909\u6570\u304c\u914d\u5217\u3067\u3042\u308b\u3053\u3068\u3092\u4f1d\u3048\u308b\u65b9\u6cd5\u3092\u78ba\u4fdd\u3059\u308b\u304b\u3089\u3001Todo\u306estate\u5909\u6570\u304cAPI\u30ea\u30af\u30a8\u30b9\u30c8\u304b\u3089\u30c7\u30fc\u30bf\u3092\u53d6\u5f97\u3059\u308b\u307e\u3067map\u95a2\u6570\u3092\u5b9f\u884c\u3057\u306a\u3044\u3088\u3046\u306b\u3057\u306a\u3051\u308c\u3070\u306a\u308a\u307e\u305b\u3093\u3002<\/p>\n\n
\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u306e\u89e3\u6c7a\u65b9\u6cd5<\/h2>\n
\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u3092\u89e3\u6c7a\u3059\u308b\u65b9\u6cd5\u306f\u3001\u4ee5\u4e0b\u306e3\u901a\u308a\u3042\u308a\u307e\u3059\u3002<\/p>\n
    \n
  1. state\u5909\u6570\u3092\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3059\u308b<\/li>\n
  2. \u6bd4\u8f03\u6f14\u7b97\u5b50\u3092\u4f7f\u7528\u3059\u308b<\/li>\n
  3. \u30aa\u30d7\u30b7\u30e7\u30ca\u30eb\u30c1\u30a7\u30fc\u30f3\uff08?.\u6f14\u7b97\u5b50\uff09\u3092\u4f7f\u7528\u3059\u308b<\/li>\n<\/ol>\n
    \u305d\u308c\u305e\u308c\u306e\u89e3\u6c7a\u65b9\u6cd5\u3092\u9806\u306b\u3054\u7d39\u4ecb\u3057\u307e\u3059\u3002<\/p>\n
    1. state\u5909\u6570\u3092\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3059\u308b<\/h3>\n
    \u307e\u305a\u624b\u59cb\u3081\u306b\u3001\u30de\u30c3\u30d4\u30f3\u30b0\u3057\u3088\u3046\u3068\u3057\u3066\u3044\u308b\u914d\u5217\u5909\u6570\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308b\u304b\u3069\u3046\u304b\u3092\u78ba\u8a8d\u3057\u307e\u3059\u3002<\/p>\n
    state\u5909\u6570\u3092\u30c7\u30d5\u30a9\u30eb\u30c8\u3067\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3059\u308b\u3053\u3068\u3067\u3001\u305d\u306e\u5909\u6570\u304c\u5e38\u306b\u5b58\u5728\u3059\u308b\u3088\u3046\u306b\u306a\u308a\u3001\u30de\u30c3\u30d4\u30f3\u30b0\u3057\u3066\u3082\u30a8\u30e9\u30fc\u304c\u8fd4\u3055\u308c\u306a\u304f\u306a\u308a\u307e\u3059\u3002<\/p>\n
    \u4f8b\u3048\u3070\u3001\u4ee5\u4e0b\u306f2\u3064\u306e\u4f3c\u305f\u3088\u3046\u306a\u30b3\u30f3\u30dd\u30fc\u30cd\u30f3\u30c8\u3067\u3059\u304c\u30011\u3064\u76ee\u306estate\u5909\u6570\u306f\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3055\u308c\u3066\u304a\u3089\u305a\u30012\u3064\u76ee\u306f\u521d\u671f\u5316\u3055\u308c\u3066\u3044\u307e\u3059\u3002<\/p>\n
    \/\/ state\u5909\u6570\u3092\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3059\u308b\u524d\nfunction MyComponent() {\n  const [myList, setMyList] = useState();\n  \n  return (\n    <ul>\n      {myList.map(item => <li>{item}<\/li>)}\n    <\/ul>\n  );\n}\n\n\/\/ state\u5909\u6570\u3092\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3057\u305f\u5f8c\nfunction MyComponent() {\n  const [myList, setMyList] = useState([]);\n\n  return (\n    <ul>\n      {myList.map(item => <li>{item}<\/li>)}\n    <\/ul>\n  );\n}<\/code><\/pre>\n
    \u3053\u306e\u4f8b\u3067\u306f\u3001useState([])<\/code>\u3092\u4f7f\u7528\u3057\u3066\u3001myList<\/code>state\u5909\u6570\u304c\u30c7\u30d5\u30a9\u30eb\u30c8\u3067\u7a7a\u306e\u914d\u5217\u306b\u306a\u3063\u3066\u3044\u307e\u3059\u3002\u3053\u308c\u306b\u3088\u308a\u3001myList<\/code>\u304c\u6700\u521d\u306f\u672a\u5b9a\u7fa9\u3067\u3042\u3063\u3066\u3082\u3001\u5e38\u306b\u914d\u5217\u3068\u306a\u308b\u305f\u3081\u3001\u30a8\u30e9\u30fc\u3092\u56de\u907f\u3059\u308b\u3053\u3068\u304c\u3067\u304d\u307e\u3059\u3002<\/p>\n
    \u30c7\u30fc\u30bf\u53d6\u5f97\u306e\u4f8b\u3068\u3057\u3066\u3001todos<\/code>state\u5909\u6570\u3092\u7a7a\u306e\u914d\u5217\uff08[]<\/code>\uff09\u306b\u521d\u671f\u5316\u3059\u308b\u3053\u3068\u3082\u53ef\u80fd\u3067\u3059\u3002<\/p>\n
    import { useState, useEffect } from 'react';\n\nfunction App() {\n  \/\/ state\u3092Todo\u306e\u7a7a\u306e\u914d\u5217\u306b\u521d\u671f\u5316\u3059\u308b\n  const [todos, setTodos] = useState([]);\n\n  useEffect(() => {\n    const getTodos = async () => {\n      const response = await fetch(\n        'https:\/\/jsonplaceholder.typicode.com\/todos?_limit=5'\n      );\n      const data = await response.json();\n      setTodos(data);\n    };\n    getTodos();\n  }, []);\n\n  console.log(todos);\n\n  return (\n    <div>\n      {todos.map((todo) => (\n        <div key={todo.id}>\n          <h2>Item: {todo.title}<\/h2>\n       <\/div>\n      ))}\n    <\/div>\n  );\n}\n\nexport default App;<\/code><\/pre>\n
    2. \u6bd4\u8f03\u6f14\u7b97\u5b50\u3092\u4f7f\u7528\u3059\u308b<\/h3>\n
    \u914d\u5217\u5909\u6570\u306b\u30de\u30c3\u30d4\u30f3\u30b0\u3059\u308b\u524d\u306b\u3001\u6bd4\u8f03\u6f14\u7b97\u5b50\u3092\u4f7f\u3063\u3066\u305d\u306e\u5909\u6570\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308b\u304b\u3069\u3046\u304b\u3092\u30c1\u30a7\u30c3\u30af\u3059\u308b\u306e\u3082\u6709\u52b9\u3067\u3059\u3002\u3053\u308c\u306b\u306f\u3001\u4e09\u9805\u6f14\u7b97\u5b50\u3084\u8ad6\u7406AND\u6f14\u7b97\u5b50\uff08&&\uff09\u3092\u4f7f\u7528\u3057\u307e\u3059\u3002<\/p>\n
    \u4ee5\u4e0b\u306f\u3001\u4e09\u9805\u6f14\u7b97\u5b50\u306e\u4f7f\u7528\u4f8b\u3067\u3059\u3002<\/p>\n
    function MyComponent() {\n  const [myList, setMyList] = useState();\n\n  return (\n    <ul>\n      {myList ? myList.map(item => <li>{item}<\/li>) : null}\n    <\/ul>\n  );\n}<\/code><\/pre>\n
    \u4e0a\u8a18\u306f\u3001myList<\/code>\u914d\u5217\u5909\u6570\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308b\u304b\u3069\u3046\u304b\u3092\u78ba\u8a8d\u3057\u3066\u304b\u3089\u3001\u5909\u6570\u306b\u30de\u30c3\u30d4\u30f3\u30b0\u3059\u308b\u4f8b\u3067\u3059\u3002myList<\/code>\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u306a\u3044\u5834\u5408\u3001\u4e09\u9805\u6f14\u7b97\u5b50\u306fnull\u3092\u8fd4\u3057\u3001\u4f55\u3082\u30ec\u30f3\u30c0\u30ea\u30f3\u30b0\u3055\u308c\u307e\u305b\u3093\u3002myList<\/code>\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308c\u3070\u3001map\u95a2\u6570\u304c\u547c\u3073\u51fa\u3055\u308c\u3001\u30ea\u30b9\u30c8\u306e\u9805\u76ee\u304c\u30ec\u30f3\u30c0\u30ea\u30f3\u30b0\u3055\u308c\u307e\u3059\u3002<\/p>\n
    \u8ad6\u7406AND\u6f14\u7b97\u5b50\u3092\u4f7f\u7528\u3057\u3066\u3082\u307b\u307c\u540c\u3058\u3067\u3059\u3002<\/p>\n
    function MyComponent() {\n  const [myList, setMyList] = useState();\n\n  return (\n    <ul>\n      {myList && myList.map(item => <li>{item}<\/li>)}\n    <\/ul>\n  );\n}<\/code><\/pre>\n
    \u4e09\u9805\u6f14\u7b97\u5b50\u306e\u3088\u3046\u306a\u6bd4\u8f03\u6f14\u7b97\u5b50\u3092\u4f7f\u7528\u3059\u308b\u3068\u3001\u8aad\u307f\u8fbc\u307f\u306e\u51e6\u7406\u3092\u884c\u3046\u3053\u3068\u304c\u3067\u304d\u308b\u305f\u3081\u3001API\u304b\u3089\u30c7\u30fc\u30bf\u3092\u53d6\u5f97\u3057\u3066\u3044\u308b\u9593\u3001\u5225\u306e\u3082\u306e\u304c\u753b\u9762\u306b\u8868\u793a\u3055\u308c\u307e\u3059\u3002<\/p>\n
    import { useState, useEffect } from 'react';\n\nfunction App() {\n  const [todos, setTodos] = useState();\n\n  useEffect(() => {\n    const getTodos = async () => {\n      const response = await fetch(\n        'https:\/\/jsonplaceholder.typicode.com\/todos?_limit=5'\n      );\n      const data = await response.json();\n      setTodos(data);\n    };\n    getTodos();\n  }, []);\n\n  console.log(todos);\n\n  return (\n   <div>\n      {todos ? (\n        todos.map((todo) => (\n          <div key={todo.id}>\n            <h2>Item: {todo.title}<\/h2>\n          <\/div>\n        ))\n      ) : (\n        <h1>Loading...<\/h1>\n      )}\n    <\/div>\n  );\n}\n\nexport default App;<\/code><\/pre>\n
    3. \u30aa\u30d7\u30b7\u30e7\u30ca\u30eb\u30c1\u30a7\u30fc\u30f3\uff08?.\u6f14\u7b97\u5b50\uff09\u3092\u4f7f\u7528\u3059\u308b<\/h3>\n
    ES2020\u3067\u5c0e\u5165\u3055\u308c\u305f\u30aa\u30d7\u30b7\u30e7\u30ca\u30eb\u30c1\u30a7\u30fc\u30f3\uff08?.\u6f14\u7b97\u5b50\uff09\u3092\u4f7f\u7528\u3059\u308b\u3068\u3001\u914d\u5217\u306emap\u95a2\u6570\u306e\u3088\u3046\u306a\u30d7\u30ed\u30d1\u30c6\u30a3\u3084\u95a2\u6570\u306b\u5b89\u5168\u306b\u30a2\u30af\u30bb\u30b9\u3067\u304d\u307e\u3059\u3002<\/p>\n
    \u4ee5\u4e0b\u306f\u3001myList<\/code>state\u5909\u6570\u3092\u78ba\u8a8d\u3059\u308b\u305f\u3081\u306b\u3001\u30aa\u30d7\u30b7\u30e7\u30ca\u30eb\u30c1\u30a7\u30fc\u30f3\u3092\u4f7f\u7528\u3059\u308b\u95a2\u6570\u30b3\u30f3\u30dd\u30fc\u30cd\u30f3\u30c8\u306e\u4f8b\u3067\u3059\u3002<\/p>\n
    function MyComponent() {\n  const [myList, setMyList] = useState();\n\n  return (\n    <div>\n      {myList?.map((item) => (\n        <p>{item}<\/p>\n      ))}\n    <\/div>\n  );\n}<\/code><\/pre>\n
    \u4e0a\u306e\u4f8b\u3067\u306f\u3001myList<\/code>\u914d\u5217\u5909\u6570\u306b\u5b89\u5168\u306b\u30a2\u30af\u30bb\u30b9\u3059\u308b\u305f\u3081\u306b\u30aa\u30d7\u30b7\u30e7\u30ca\u30eb\u30c1\u30a7\u30fc\u30f3\u304c\u4f7f\u7528\u3055\u308c\u3066\u3044\u307e\u3059\u3002myList<\/code>\u304c\u672a\u5b9a\u7fa9\u306e\u5834\u5408\u3001\u4f55\u3082\u30ec\u30f3\u30c0\u30ea\u30f3\u30b0\u3055\u308c\u305a\u3001myList<\/code>\u304c\u5b9a\u7fa9\u3055\u308c\u3066\u3044\u308c\u3070\u3001map\u95a2\u6570\u304c\u547c\u3073\u51fa\u3055\u308c\u3001\u30ea\u30b9\u30c8\u9805\u76ee\u304c\u30ec\u30f3\u30c0\u30ea\u30f3\u30b0\u3055\u308c\u307e\u3059\u3002<\/p>\n\n
    \u307e\u3068\u3081<\/h2>\n
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    \u300cTypeError: Cannot read pr …<\/p>\n","protected":false},"author":287,"featured_media":45296,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_kinsta_gated_content":false,"_kinsta_gated_content_redirect":"","footnotes":""},"tags":[],"topic":[802,803],"class_list":["post-45295","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","topic-react","topic-react-errors"],"yoast_head":"\nReact\u306e\u300cTypeError: Cannot read property \u2018map\u2019 of undefined\u300d\u30a8\u30e9\u30fc\u306e\u89e3\u6c7a\u65b9\u6cd5\uff5cKinsta\u00ae<\/title>\n<meta name=\"description\" content=\"\u30a4\u30e9\u30a4\u30e9\u306e\u7a2e\u306b\u306a\u308b\u300cTypeError: Cannot read property \u2018map\u2019 of 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