Javascript “this”在地图函数 Reactjs 中未定义
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"this" is undefined inside map function Reactjs
提问by iamhuy
I'm working with Reactjs, writing a menu component.
我正在使用 Reactjs，编写菜单组件。
"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.
每当我在 map 函数内部使用 'this' 时，它都是未定义的，但在外部没有问题。
The error:
错误：
"Cannot read property 'props' of undefined"
“无法读取未定义的属性‘道具’”
Anybody help me ! :((
任何人都帮助我！:((
回答by Jonny Buchanan
Array.prototype.map()takes a second argument to set what thisrefers to in the mapping function, so pass thisas the second argument to preserve the current context:
Array.prototype.map()需要第二个参数来设置this映射函数中引用的内容，因此this作为第二个参数传递以保留当前上下文：
someList.map(function(item) {
...
}, this)
Alternatively, you can use an ES6 arrow functionto automatically preserve the current thiscontext:
或者，您可以使用 ES6箭头函数自动保留当前this上下文：
someList.map((item) => {
...
})
