map不是jsx中的函数

Question

This block of code caused error of map is not a function

 {data && (data.devices || {}).map((obj, i) => 
    <div>{obj.name}</div>
 )}

I just don't get it, I already did data && to check data is defined else keep the map. And also data.devices || {} data.devices || {} to check if the devices property is there or not.

I console.log(data.devices) is return undefined but it should fallback to an object right? why is it still breaking?

Answer 1

There is no native .map to {} , so replace data.devices || {} data.devices || {} to data.devices || [] data.devices || []

{(data && data.devices || []).map((obj, i) => 
    <div>{obj.name}</div>
)}

Answer 2

In this case map is not a function because when data.devices is empty, the default value is an empty object hence map is not a function of an object. Take this for example:

 // simulate scenarios const data00 = undefined; const data01 = {}; const data02 = { devices: null }; const data03 = { devices: [] }; const data04 = { devices: [{ name: 'device01' }, { name: 'device02' }] } class Main extends React.Component { render() { return ( <div> {this.load(data00)} {this.load(data01)} {this.load(data02)} {this.load(data03)} {this.load(data04)} </div> ); } /** * Loads data and renders the devices if there's any */ load(data) { return (!!data && data.devices || []).map((obj, i) => <div key={i}>{obj.name}</div> ); } } ReactDOM.render(<Main />, document.getElementById('main')); 

 <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script> <div id="main"/> 

As you can see, only data04 's devices will be rendered.