Mystery of "this" in ReactJS - facebook

Question

I am fairly new to the Facebook's React world. Their documentation seems to be very good but there are a few areas where I need a little bit of clarity. This is one of them.
Src: http://tuts-javascript.appspot.com/reactjs-add-remove-table-row
var CompanyApp = React.createClass({
getInitialState: function() {
return {companylist:this.props.companies};
},
handleNewRowSubmit: function( newcompany ) {
this.setState( {companylist: this.state.companylist.concat([newcompany])} );
},
handleCompanyRemove: function( company ) {
var index = -1;
var clength = this.state.companylist.length;
for( var i = 0; i < clength; i++ ) {
if( this.state.companylist[i].cname === company.cname ) {
index = i;
break;
}
}
this.state.companylist.splice( index, 1 );
this.setState( {companylist: this.state.companylist} );
},
render: function() {
var tableStyle = {width: '100%'};
var leftTdStyle = {width: '50%',padding:'20px',verticalAlign: 'top'};
var rightTdStyle = {width: '50%',padding:'20px',verticalAlign: 'top'};
return (
<table style={tableStyle}>
<tr>
<td style={leftTdStyle}>
<CompanyList clist={this.state.companylist} onCompanyRemove={this.handleCompanyRemove}/>
</td>
<td style={rightTdStyle}>
<NewRow onRowSubmit={this.handleNewRowSubmit}/>
</td>
</tr>
</table>
);
}
});
var CompanyList = React.createClass({
handleCompanyRemove: function(company){
this.props.onCompanyRemove( company );
},
render: function() {
var companies = [];
var that = this; // TODO: Needs to find out why that = this made it work; Was getting error that onCompanyDelete is not undefined
this.props.clist.forEach(function(company) {
companies.push(<Company company={company} onCompanyDelete={that.handleCompanyRemove} /> );
});
return (
<div>
<h3>List of Companies</h3>
<table className="table table-striped">
<thead><tr><th>Company Name</th><th>Employees</th><th>Head Office</th><th>Action</th></tr></thead>
<tbody>{companies}</tbody>
</table>
</div>
);
}
});
var Company = React.createClass({
handleRemoveCompany: function() {
this.props.onCompanyDelete( this.props.company );
return false;
},
render: function() {
return (
<tr>
<td>{this.props.company.cname}</td>
<td>{this.props.company.ecount}</td>
<td>{this.props.company.hoffice}</td>
<td><input type="button" className="btn btn-primary" value="Remove" onClick={this.handleRemoveCompany}/></td>
</tr>
);
}
});
var NewRow = React.createClass({
handleSubmit: function() {
var cname = this.refs.cname.getDOMNode().value;
var ecount = this.refs.ecount.getDOMNode().value;
var hoffice = this.refs.hoffice.getDOMNode().value;
var newrow = {cname: cname, ecount: ecount, hoffice: hoffice };
this.props.onRowSubmit( newrow );
this.refs.cname.getDOMNode().value = '';
this.refs.ecount.getDOMNode().value = '';
this.refs.hoffice.getDOMNode().value = '';
return false;
},
render: function() {
var inputStyle = {padding:'12px'}
return (
<div className="well">
<h3>Add A Company</h3>
<form onSubmit={this.handleSubmit}>
<div className="input-group input-group-lg" style={inputStyle}>
<input type="text" className="form-control col-md-8" placeholder="Company Name" ref="cname"/>
</div>
<div className="input-group input-group-lg" style={inputStyle}>
<input type="text" className="form-control col-md-8" placeholder="Employee Count" ref="ecount"/>
</div>
<div className="input-group input-group-lg" style={inputStyle}>
<input type="text" className="form-control col-md-8" placeholder="Headoffice" ref="hoffice"/>
</div>
<div className="input-group input-group-lg" style={inputStyle}>
<input type="submit" className="btn btn-primary" value="Add Company"/>
</div>
</form>
</div>
);
}
});
var defCompanies = [{cname:"Infosys Technologies",ecount:150000,hoffice:"Bangalore"},{cname:"TCS",ecount:140000,hoffice:"Mumbai"}];
React.renderComponent( <CompanyApp companies={defCompanies}/>, document.getElementById( "companyApp" ) );
This is a very good basic explanation of how ReactJS works. Thanks to the author.
But this comment,
var that = this; // TODO: Needs to find out why that = this made it work; Was getting error that onCompanyDelete is not undefined
Why is that necessary?
Is this the right way to do it? If not, what is?
Thanks in advance.

Answer 1

There's no mystery of "this" that is specific to ReactJS.
This is just a case of standard scoping issues that crop up with callbacks in JavaScript.
When you're in a react component, all methods on the base component will be scoped with the this being the current component, just like any other JavaScript "class".
In your snippet you have a render method which is a function on the base component and therefore this is equal to the component itself. However within that render method you're calling a callback with this.props.clist.forEach, any function callbacks inside the render method will need to be either bound to the correct this scope, or you can do var that = this (although this is an anti-pattern and should be discouraged)`.
Example, slightly simplified version of your snippet:
var MyComponent = React.createClass({
handleCompanyRemove: function(e) {
// ...
},
render: function() {
// this === MyComponent within this scope
this.props.someArray.forEach(function(item) {
// this !== MyComponent, therefore this.handleCompanyRemove cannot
// be called!
})
}
})
As you can see from the comments above, inside your callback for the .forEach you cannot use this directly without either defining a variable outside, or properly binding the function.
Other options to solve this are:
Binding the callback function to the correct this scope. Example:
this.props.someArray.forEach(function(item) {
// this === MyComponent within this scope too now!
// so you can call this.handleCompanyRemove with no problem
}.bind(this))
If you're using Babel/ES6 you can use the Fat Arrow function syntax which guarantees that this scope continues to the callback from the parent scope. Example:
this.props.someArray.forEach((item) => {
// this === MyComponent within this scope too now!
// so you can call this.handleCompanyRemove with no problem
})