“this” is undefined inside map function Reactjs

问题

I\'m working with Reactjs, writing a menu component.

\"use strict\";

var React = require(\"react\");
var Menus = React.createClass({

    item_url: function (item,categories,articles) {
        console.log(\'afdasfasfasdfasdf\');
        var url=\'XXX\';
        if (item.type == 1) {
            url = item.categoryId == null ? \'javascript:void(0)\' : path(\'buex_portal_browse_category\', {slug: categories[item.categoryId].slug});
        } else if (item.type == 2) {
            url = item.articleId == null ? \'javascript:void(0)\' : path(\'buex_portal_view_article\', {slug: articles[item.articleId].slug, id: item.articleId});
        } else {
            url = item.url;
        }
        return url;
    },

    render: function () {
     //   console.log(this.props.menus);  // return correctly
            var menuElements = this.props.menus.map(function (item1) { // return fault : \'cannot read property \'props\' of undefined \'
            return (
                <div>
                    <li>
                        <a href={this.item_url(item1, this.props.categories, this.props.articles )}>{item1.name} // the same fault above
                            <i class=\"glyphicon glyphicon-chevron-right pull-right\"></i>
                        </a>
                        <div class=\"sub-menu\">
                            <div>
                            {item1._children.map(function (item2) {
                                return (
                                    <div>
                                        <h4>
                                            <a href={this.item_url(item2, this.props.categories, this.props.articles)}>{ item2.name }</a>
                                        </h4>
                                        <ul>
                                            {item2._children.map(function (item3) {
                                                return ( 
                                                    <div><li><a href={this.item_url(item3, this.props.categories, this.props.articles) }>{ item3.name }</a></li></div>
                                                );
                                            })}                   
                                        </ul>
                                    </div>
                                );
                            })}
                            </div>
                        </div>
                    </li>
                </div>
            );
        });

        return (
            <div class=\"menu\">
                <ul class=\"nav nav-tabs nav-stacked\">
                    {menuElements}
                </ul>
            </div>
        );
    }
});

Whenever I use \'this\' inside map function it is undefined, but outside it is no problem.

The error:

\"Cannot read property \'props\' of undefined\"

Anybody help me ! :((

回答1:

Array.prototype.map() takes a second argument to set what this refers to in the mapping function, so pass this as the second argument to preserve the current context:

someList.map(function(item) {
  ...
}, this)

Alternatively, you can use an ES6 arrow function to automatically preserve the current this context:

someList.map((item) => {
  ...
})

来源：https://stackoverflow.com/questions/30148827/this-is-undefined-inside-map-function-reactjs

标签

javascript

reactjs

this

map-function