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[英]how to set first value of array as default in cascading dropdown in react hooks
[英]How to set the first value in a select dropdown to as a default react
我有一个反应程序,它根据下拉列表的值显示一个表格。 我希望程序默认根据下拉列表中的第一个值显示表格。
下拉列表中的第一个值与默认值有很大不同,并且下拉列表值总是在变化。 因此,数据在首次加载时可能会产生误导。
这是我的代码片段,里面有一些描述。 谢谢。
const CompletenessApp = () => {
const [completeness, setcompleteness] = useState([]);
const [loading, setloading] = useState(false);
const [valueSelected, setValueSelected] = useState({value:'all'});
const [periodSelected, setPeriodSelected] = useState({value:'20200702'}); // the default value that is used to filter the data.
const valid = [
{id:"all", variable:"all"},{id:"true", variable:"true"}, {id:"false", variable:"false"}, {id:"null", variable:"null"},
];
useEffect(() => {
const fetchData = async () => {
try{
const res = await axios.get('http://localhost:8000/api/completeness/');
setcompleteness(res.data);
setloading(true);
} catch (e) {
console.log(e)
}
}
fetchData();
}, []);
// when the page loads for the first time it filters the data based on the period value set in the state. I want the data to be filtered based on the first value in the dropdown instead, the first value in the dropdown is different from the default value set.
const handlePeriodChange = e => {
setPeriodSelected({value : e.target.value})
}
const handleValueChange = e => {
let boolvalue = Boolean
e.target.value === 'true'? boolvalue = true:
e.target.value === 'false'? boolvalue = false:
e.target.value === 'all'? boolvalue = 'all':
boolvalue=null
setValueSelected({value : boolvalue})
}
//filtered data to be displayed
const filteredCompleteness = completeness.filter(
completedata=> (completedata.period === periodSelected.value)
&&(valueSelected.value !== 'all'?completedata.complete === valueSelected.value:{}))
return(
<div>
<div className="selection-row">
<div className="stats-columns">
<div className="stats-label">Period</div>
//the relevant dropdown is here
<select onChange={e => handlePeriodChange(e)}>
{Array.from(new Set(completeness.map(obj => obj.period)))
.sort().reverse()
.map(period => {
return <option value={period}>{period}</option>
})}
</select>
</div>
<div className="stats-columns">
<div className="stats-label">Value</div>
<select onChange={e => handleValueChange(e)}>
{valid.map((obj) => {
return <option value={obj.id}>{obj.variable}</option>
})
}
</select>
</div>
</div>
<hr></hr>
<div className="results-table">
//table is displayed here
</div>
</div>
);
}
export default CompletenessApp;
// above return
const options = Array.from(new Set(completeness.map((obj) => obj.period)))
.sort()
.reverse()
.map((period) => {
return {
value: period,
label: period,
};
});
// in jsx
<select defaultValue={options[0].value} onChange={e => handlePeriodChange(e)}>
{
options.map((obj) => {
return <option value={obj.value}>{obj.label}</option>;
})
}
</select>
试试这个,让我知道。
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