ReactJS can't access "this" methods - reactjs

Question

I am trying to pass a method to a child component to handle onclick events.
I saw a lot of examples online, but I can't get it working.
When I am inside the render function of the parent and trying to pass "this.handleClick" to the child, handleClick is undefined.
Have a look at render method of ThumbList:
var Thumb = React.createClass({
handleClick: function() {
console.log(this)
console.log('handleClick of Thumb')
this.props.onClick()
},
render: function() {
return(
<div className="thumb" key={this.props.thumb.url}>
<a href='#' onClick={this.handleClick}>
<img src={'/img/rings/thumbs/'+this.props.thumb.url+'_thumb.jpg'} alt="Image">
</img>
</a>
</div>
);
}
});
var ThumbList = React.createClass({
handleClick: function (id) {
console.log('click of ThumbList');
},
loadFromServer: function() {
$.ajax({
url: 'rings/imgs/5',
dataType: 'json',
success: function(data) {
this.setState({data: data});
}.bind(this),
error: function(xhr, status, err) {
console.error('rings/imgs/5', status, err.toString());
}.bind(this)
});
},
getInitialState: function(){
return {data: [] };
},
componentDidMount: function(){
this.loadFromServer();
setInterval(this.loadFromServer, 2000);
},
render: function() {
var handlefunc=this.handleClick
var thumbsNodes = this.state.data.map(function(thumb) {
console.log(this.handleClick) // is Undefined!
console.log(handlefunc) // is working
return (
<Thumb thumb={thumb} key={thumb.url} onClick={handlefunc.bind(this,thumb.url)}/>
);
});
return(
<div className="col-md-1 col-md-offset-1" id='thumbs'>
{thumbsNodes}
</div>
);
}
});
Any idea what I might be missing?

Answer 1

If you're using a compiler like Babel as part of your development workflow, I'd suggest using arrow functions:
var thumbsNodes = this.state.data.map((thumb) => {
console.log(this.handleClick);
return <Thumb thumb={thumb} key={thumb.url}
onClick={this.handlefunc.bind(this,thumb.url)}/>;
});
As you can see, it's a nice compact syntax. The arrow function will preserve the this context for you. The Babel compiler produces JavaScript that uses a closure:
var thumbsNodes = this.state.data.map(function(thumb) {
var _this = this;
console.log(_this.handleClick);
return <Thumb thumb={thumb} key={thumb.url}
onClick={_this.handlefunc.bind(_this,thumb.url)}/>;
});

Answer 2

this is undefined because the map callback does not know what it is. The simplest way to solve this is to pass a second argument, and it will use that as this in the callback:
var thumbsNodes = this.state.data.map(function(thumb) {
console.log(this.handleClick)
return <Thumb thumb={thumb} key={thumb.url} onClick={handlefunc.bind(this,thumb.url)}/>
}, this)
More: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map

Answer 3

You need to grab a reference to this so it is the correct context when you call it.
Try
render: function()
{
var handlefunc = this.handleClick; // i assume this is just for debugging the issue
var self = this;
var thumbsNodes = this.state.data.map(function(thumb)
{
console.log(self.handleClick) // note the use of `self`
});
}