score:1

I believe you have already found what you were looking for but, I saw this question but no answer, still.

You just need to define a variable outside mapping and while mapping, put an if condition to find what you looking for and then equalize the result to the variable outside.

Like,

    let tmpItem;
    this.state.newOrder.map((mapItem) => {
        if (mapItem.id == "99") {
            console.log(mapItem)
            tmpItem = mapItem;
        }
    });

score:2

It's better to use find, but it is not supported in all browsers. Use reduce instead of map:

var SelectedTopicPage = React.createClass({

    render: function() {
        var selectedId = this.props.selectedId; // id what you need to find
        var topicPage = this.props.topicPages.reduce(function(topic, current) {
            return topic.topic_no == selectedId ? current : topic;
        }, undefined);

        return (
            <SelectedTopicPageMarkup headline={topicPage.headline} key={topicPage.topic_no}>
                {topicPage.description}
            </SelectedTopicPageMarkup>
        );
    }
});

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