ReactJS map function cannot find property of undefined

Question

I\'m still learning ReactJS. I\'m challenging myself to write a very basic todo app (as one does) and I\'m having an issue calling an onClick function.

var L         


        

Answer 1

You should bind this explicitly to the handleClick function to be referring to the React component not the map function, so you can refactor your code as follow:

var items = list.map(renderListItem.bind(this));

And add renderListItem method in your React class as follow:

renderListItem(item) {
  return (
    <li style={{borderBottom:'1px solid red'}}>
      <label onClick={this.handleClick}>
        <input type="checkbox" />
        {item}
      </label>
    </li>
  );
}

Answer 2

The second argument for the map function is a value to define the scope of this when executing the callback.:

.map( callback( currentValue, index, array), value_for_this/scope_to_run_in )

So you can modify your map function as follows:

var items = list.map(function(item){
  return (
    <li style={{borderBottom:'1px solid red'}}>
      <label onClick={this.handleClick}>
        <input type="checkbox" />
        {item}
      </label>
    </li>
  );
}, this);

---

You could also use an arrow function which where this is implicitly bound:

var items = list.map((item) => {
  return (
    <li style={{borderBottom:'1px solid red'}}>
      <label onClick={this.handleClick}>
        <input type="checkbox" />
        {item}
      </label>
    </li>
  );
});

Answer 3

The problem you're running into is that your call to list.map will invoke the passed function with a different this than you have in your render method.

An easy fix is to grab this in the outer scope and stash it in a variable, then use that variable in your inline function.

render: function () {
    var self = this;
 // ^^^^^^^^^^^^^^^^

    var list = this.props.items;
    var items = list.map(function(item){
      return (
        <li style={{borderBottom:'1px solid red'}}>
          <label onClick={self.handleClick}>
                       // ^^^^
            <input type="checkbox" />
            {item}
          </label>
        </li>
      );
    });

    return (
      <ul>{items}</ul>
    )
}