為什么我無法訪問狀態的嵌套數據
[英]why I can't access the nested data in state
問題描述
我正在嘗試訪問存儲在this.state.data中的嵌套數組和對象。 但是我無法訪問陣列，誰能告訴我為什么？ 任何人都為什么會不斷彈出錯誤“無法讀取未定義的屬性'0'”。 這個錯誤使我發狂。 我不明白為什么這種情況會持續下去。
import React from 'react';
const url="https://pixabay.com/api/?key=13565216-b3f251bf75153fd651dec947c&q=yellow+flowers&image_type=photo&pretty=true";
class App extends React.Component{
constructor(props){
super(props);
this.state = {
input : '',
data: '',
isloading: true
}
this.fetchData=this.fetchData.bind(this);
}
componentDidMount(){
this.fetchData();
}
fetchData(){
fetch(url)
.then(res=>res.json())
.then(result=>{
this.setState({
data:result,
isloading: false
})
});
}
render(){
return(
<>
{
console.log(this.state.data.hits[0])
}
</>
);
}
}
export default App;
json數據在這里：
{
"totalHits":500,
"hits":[
{
"largeImageURL":"https://pixabay.com/get/55e0d340485aa814f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":426,
"webformatWidth":640,
"likes":1008,
"imageWidth":6000,
"id":3063284,
"user_id":1564471,
"views":651677,
"comments":226,
"pageURL":"https://pixabay.com/photos/rose-flower-petal-floral-noble-3063284/",
"imageHeight":4000,
"webformatURL":"https://pixabay.com/get/55e0d340485aa814f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":99,
"tags":"rose, flower, petal",
"downloads":416283,
"user":"annca",
"favorites":873,
"imageSize":3574625,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2015/11/27/06-58-54-609_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2018/01/05/16/24/rose-3063284_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/55e1d4404953a414f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":426,
"webformatWidth":640,
"likes":788,
"imageWidth":2736,
"id":3113318,
"user_id":7410713,
"views":433603,
"comments":123,
"pageURL":"https://pixabay.com/photos/sunflower-nature-flora-flower-3113318/",
"imageHeight":1824,
"webformatURL":"https://pixabay.com/get/55e1d4404953a414f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":99,
"tags":"sunflower, nature, flora",
"downloads":318174,
"user":"bichnguyenvo",
"favorites":526,
"imageSize":1026006,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2017/12/16/10-28-39-199_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2018/01/28/11/24/sunflower-3113318_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/54e2dc464e51a814f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":426,
"webformatWidth":640,
"likes":789,
"imageWidth":5363,
"id":2295434,
"user_id":334088,
"views":127443,
"comments":60,
"pageURL":"https://pixabay.com/photos/spring-bird-bird-tit-spring-blue-2295434/",
"imageHeight":3575,
"webformatURL":"https://pixabay.com/get/54e2dc464e51a814f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":99,
"tags":"spring bird, bird, tit",
"downloads":56295,
"user":"JillWellington",
"favorites":880,
"imageSize":2938651,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2018/06/27/01-23-02-27_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2017/05/08/13/15/spring-bird-2295434_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/55e2dc414351ae14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":400,
"webformatWidth":640,
"likes":733,
"imageWidth":3200,
"id":3292932,
"user_id":2216431,
"views":366876,
"comments":65,
"pageURL":"https://pixabay.com/photos/sunflower-vase-vintage-retro-wall-3292932/",
"imageHeight":2000,
"webformatURL":"https://pixabay.com/get/55e2dc414351ae14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":93,
"tags":"sunflower, vase, vintage",
"downloads":283271,
"user":"Yuri_B",
"favorites":946,
"imageSize":2563708,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2018/01/15/10-52-15-382_250x250.png",
"previewURL":"https://cdn.pixabay.com/photo/2018/04/05/14/09/sunflower-3292932_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/51e1d0464e52b108f5d084609629347b133adce4554c704c73267fd79f4cc05a_1280.jpg",
"webformatHeight":360,
"webformatWidth":640,
"likes":295,
"imageWidth":3020,
"id":715540,
"user_id":916237,
"views":96804,
"comments":33,
"pageURL":"https://pixabay.com/photos/yellow-natural-flower-blossom-715540/",
"imageHeight":1703,
"webformatURL":"https://pixabay.com/get/51e1d0464e52b108f5d084609629347b133adce4554c704c73267fd79f4cc05a_640.jpg",
"type":"photo",
"previewHeight":84,
"tags":"yellow, natural, flower",
"downloads":48832,
"user":"Wow_Pho",
"favorites":309,
"imageSize":974940,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2015/04/07/14-10-15-590_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2015/04/10/00/41/yellow-715540_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/57e5d4414253af14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":323,
"webformatWidth":640,
"likes":294,
"imageWidth":3861,
"id":1512813,
"user_id":2364555,
"views":82697,
"comments":23,
"pageURL":"https://pixabay.com/photos/lily-flowers-early-flower-garden-1512813/",
"imageHeight":1952,
"webformatURL":"https://pixabay.com/get/57e5d4414253af14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":75,
"tags":"lily, flowers, early",
"downloads":40483,
"user":"pixel2013",
"favorites":305,
"imageSize":1037708,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/07/15/18-51-07-612_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2016/07/12/18/54/lily-1512813_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/54e1d1464f51a514f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
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"webformatWidth":640,
"likes":413,
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"id":2145539,
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"views":53994,
"comments":31,
"pageURL":"https://pixabay.com/photos/crocus-flower-wet-spring-2145539/",
"imageHeight":2443,
"webformatURL":"https://pixabay.com/get/54e1d1464f51a514f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":91,
"tags":"crocus, flower, wet",
"downloads":31006,
"user":"pixel2013",
"favorites":388,
"imageSize":823922,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/07/15/18-51-07-612_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2017/03/15/09/00/crocus-2145539_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/57e5d6454a5aa414f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":419,
"webformatWidth":640,
"likes":296,
"imageWidth":4896,
"id":1536088,
"user_id":1195798,
"views":290456,
"comments":57,
"pageURL":"https://pixabay.com/photos/sunflower-flower-bloom-yellow-1536088/",
"imageHeight":3208,
"webformatURL":"https://pixabay.com/get/57e5d6454a5aa414f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":98,
"tags":"sunflower, flower, bloom",
"downloads":51793,
"user":"Couleur",
"favorites":269,
"imageSize":5103399,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/07/30/15-24-04-643_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2016/07/23/00/12/sun-flower-1536088_150.jpg"
},
{
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"likes":375,
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"id":3215188,
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"comments":98,
"pageURL":"https://pixabay.com/photos/flowers-orange-orange-petals-3215188/",
"imageHeight":1134,
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"previewHeight":83,
"tags":"flowers, orange, orange petals",
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"user":"Candiix",
"favorites":314,
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"userImageURL":"https://cdn.pixabay.com/user/2017/12/06/20-08-45-84_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2018/03/10/20/26/flowers-3215188_150.jpg"
},
{
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"likes":334,
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"user_id":909086,
"views":48785,
"comments":21,
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"imageHeight":1288,
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"type":"photo",
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"tags":"flower, beautiful, bloom",
"downloads":31414,
"user":"Bessi",
"favorites":429,
"imageSize":370390,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/04/11/22-45-05-994_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2015/04/19/08/33/flower-729515_150.jpg"
},
{
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"likes":538,
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"id":56414,
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"views":77521,
"comments":82,
"pageURL":"https://pixabay.com/photos/anemone-flower-blossom-bloom-blue-56414/",
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"webformatURL":"https://pixabay.com/get/53e6d1424e4fad0bffd8992cc62b3278103ddfe34e50744f7c2b78dc9648c7_640.jpg",
"type":"photo",
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"tags":"anemone, flower, blossom",
"downloads":32632,
"user":"Albenheim",
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"imageSize":770723,
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"userImageURL":"https://cdn.pixabay.com/user/2012/09/08/21-14-56-990_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2012/09/08/21/51/anemone-56414_150.jpg"
},
{
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},
{
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"tags":"rose, flower, yellow",
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"previewURL":"https://cdn.pixabay.com/photo/2013/05/26/12/14/rose-113735_150.jpg"
},
{
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"comments":35,
"pageURL":"https://pixabay.com/photos/drip-blossom-bloom-yellow-1972411/",
"imageHeight":2499,
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"type":"photo",
"previewHeight":87,
"tags":"drip, blossom, bloom",
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"user":"susannp4",
"favorites":285,
"imageSize":1510459,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2015/12/16/17-56-55-832_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2017/01/11/17/27/drip-1972411_150.jpg"
},
{
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"webformatHeight":497,
"webformatWidth":640,
"likes":429,
"imageWidth":3867,
"id":887443,
"user_id":1298145,
"views":82485,
"comments":49,
"pageURL":"https://pixabay.com/photos/flower-life-crack-desert-drought-887443/",
"imageHeight":3005,
"webformatURL":"https://pixabay.com/get/5ee8d2474e51b108f5d084609629347b133adce4554c704c73267fd79f4cc05a_640.jpg",
"type":"photo",
"previewHeight":116,
"tags":"flower, life, crack",
"downloads":41067,
"user":"klimkin",
"favorites":351,
"imageSize":2611531,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2017/07/18/13-46-18-393_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2015/08/13/20/06/flower-887443_150.jpg"
},
{
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"webformatWidth":640,
"likes":186,
"imageWidth":6000,
"id":3720383,
"user_id":6246704,
"views":59481,
"comments":42,
"pageURL":"https://pixabay.com/photos/flower-g%C3%A9rbel-yellow-flower-flower-3720383/",
"imageHeight":4000,
"webformatURL":"https://pixabay.com/get/55e7d743495aaf14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":99,
"tags":"flower g\u00e9rbel, yellow flower, flower",
"downloads":48052,
"user":"fernandozhiminaicela",
"favorites":160,
"imageSize":2117262,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/02/27/14-16-13-192_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2018/10/03/03/42/flower-gerbel-3720383_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/55e6d1434351a914f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":426,
"webformatWidth":640,
"likes":339,
"imageWidth":5574,
"id":3640935,
"user_id":334088,
"views":176477,
"comments":63,
"pageURL":"https://pixabay.com/photos/sunflowers-field-woman-yellow-3640935/",
"imageHeight":3717,
"webformatURL":"https://pixabay.com/get/55e6d1434351a914f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_640.jpg",
"type":"photo",
"previewHeight":99,
"tags":"sunflowers, field, woman",
"downloads":145783,
"user":"JillWellington",
"favorites":308,
"imageSize":4970597,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2018/06/27/01-23-02-27_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2018/08/29/22/52/sunflowers-3640935_150.jpg"
},
{
"largeImageURL":"https://pixabay.com/get/57e6d7444b5baf14f6da8c7dda79367d1039dbe752576c48702972d09445c45dba_1280.jpg",
"webformatHeight":426,
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"id":1627193,
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"comments":42,
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},
{
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"imageHeight":3760,
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"type":"photo",
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"user":"pixel2013",
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"previewURL":"https://cdn.pixabay.com/photo/2018/07/15/19/02/sun-flower-3540266_150.jpg"
},
{
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"webformatHeight":426,
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"id":1319114,
"user_id":485024,
"views":99904,
"comments":25,
"pageURL":"https://pixabay.com/photos/girl-flowers-yellow-beauty-nature-1319114/",
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"type":"photo",
"previewHeight":99,
"tags":"girl, flowers, yellow",
"downloads":48347,
"user":"AdinaVoicu",
"favorites":292,
"imageSize":3837334,
"previewWidth":150,
"userImageURL":"https://cdn.pixabay.com/user/2019/07/18/16-54-09-399_250x250.jpg",
"previewURL":"https://cdn.pixabay.com/photo/2016/04/09/23/10/girl-1319114_150.jpg"
}
],
"total":21247
}
5 個解決方案
解決方案1
2 2019-09-25 19:16:04
問題是您處於狀態的初始值為data: '' ，這是一個字符串值，在執行this.state.data.hits[0]時在第一次渲染期間會引發錯誤。
要解決此問題，您可以將初始狀態更改為如下所示
this.state = {
input : '',
data: {
hits: []
,
isloading: true
}
建議：
- 您不需要綁定方法
fetchData因為它在同一作用域中被調用
如果刪除此綁定，則甚至不需要構造函數，可以用以下代碼替換整個構造函數
state = {
input : '',
data: { hits: [] },
isloading: true
}
解決方案2
1 2019-09-25 19:17:21
在Innitial渲染過程中， data為空字符串，並且試圖訪問hits[0]實際上是未定義的，this.state.data.hits本身是未定義的，因此您可以首先在訪問它之前進行檢查，如下所示
console.log( this.state.data.hits && this.state.data.hits[0])
解決方案3
1 已采納 2019-09-25 19:18:32
（有關有條件調用console.log（）還是創建與您期望的數據結構相匹配的初始狀態更好的方法，請參見下面的有趣評論。此代碼確實有效，盡管有些人喜歡采用其他方法）。
在獲取數據之前，render方法正在運行，因此此時，您的數據等於一個空字符串。
您可以放置條件語句，以便僅在存儲數據時才執行console.log（）：
render(){
return(
<div>
{
this.state.data && this.state.data.hits && console.log(this.state.data.hits[0])
}
</div>
);
}
如果沒有this.state.data，則代碼不會超過下一個&&運算符。 但是，您還需要檢查數據對象是否具有“ hits”屬性，因此您需要為此編寫另一個&&運算符。 Console.log（）並非“虛假”，因此它將運行。
解決方案4
0 2019-09-25 19:19:53
this.fetchData()是異步的，這意味着發送請求（或接收響應） this.fetchData()正常執行流中刪除。 因此， render()函數在數據接收之前調用。
解
檢查this.state.data或this.state.data.hits是否為null。
{this.state.data.hits && console.log(this.state.data.hits[0])}
解決方案5
0 2019-09-25 19:24:19
在indside構造函數中，您已將數據定義為字符串並分配了對象數組。 小變化： data: '' to data: []
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