ReactJS: TypeError: Cannot read property 'map' of undefined

Question

I get this error in my ReactJS app that pulls places data. it seems to me that the error shows when map() points to null

TypeError: Cannot read property 'map' of undefined

I could not figure out a way to make a default value

and this is the code:

import React, {Component} from 'react';
// This component is for search list view, it render the props places data,
// And handle cilck for place item.
class SearchList extends Component {
    render() {
        // const {places, query, selectPlace} = this.props;
        const {places,query,selectPlace} = this.props;


        return (

        <div className="container">
                <hr/>
                <div className="input-group">
                    <input
                        type="text"
                        className="form-control"
                        placeholder="Filter"
                        aria-label="Filter Input"
                        onChange={(event) => {
                            query(event.target.value);
                        }}
                    />
                    <span className="input-group-addon">
              <i className="fas fa-filter"></i>
            </span>
                </div>
                <hr/>
                <div style={{maxHeight: '82vh', overflow: 'scroll'}}>
                    <ul className="list-group">
                        {
                            places.map(place => (
                            <li
                                tabIndex="0"
                                key={place.id}
                                className="list-group-item list-group-item-action"
                                onClick={() => selectPlace(place)}>
                                <span>{place.name}</span>
                            </li>
                        ))
                        }
                    </ul>
                </div>
            </div>
        );
    }
}

export default SearchList;

Answer 1

One can use the conditional rendering to render your places. Therefore if places is undefined the right operand of the and operator (&&) will not be rendered

                 <ul className="list-group">
                    {
                       places && places.map(place => (
                        <li
                            tabIndex="0"
                            key={place.id}
                            className="list-group-item list-group-item-action"
                            onClick={() => selectPlace(place)}>
                            <span>{place.name}</span>
                        </li>
                    ))
                    }
                </ul>

Answer 2

This happens on the first render since most likely your places comes from a resolved Promise somewhere which makes that prop undefined. There is a way to fix this, defaultProps.

SearchList.defaultProps = {
    places: [],

}

However, if you're passing places, like <SearchList places={this.state.places} /> you need to be sure that this.state.places is undefined and not null since a null value is valid and will override your defaultProps. If you want to guard against that then you need to do the conditional rendering.

places && places.map().