Why is React\'s `this.setState` Function Unavailable in ComponentDidMount Callback?

"React this.setState is not a function" Issue: Contextual Binding

When attempting to develop an application utilizing a third-party API, a common error encountered is "TypeError: this.setState is not a function." This arises when attempting to handle API responses. The problem typically stems from improper binding.

The provided React code defines a component called AppMain with a componentDidMount method. In this method, a VK API call is made to retrieve user information. Within the callback function of the API response, an attempt is made to use this.setState to update the component's state. However, the error occurs because the callback function is executed in a different context and does not have access to this.setState.

Solution:

To resolve this issue, the callback function must be bound to the correct context. This can be achieved by using the .bind method:

VK.api('users.get', { fields: 'photo_50' }, function(data) {
  if (data.response) {
    this.setState({ // the error happens here
      FirstName: data.response[0].first_name
    });
    console.info(this.state.FirstName);
  }
}.bind(this));

By binding the callback function to the component instance, the this keyword within the callback function will refer to the component instance, allowing access to its methods and properties, including this.setState.

Additional Binding:

In some cases, additional binding may be necessary. For example, if the VK init method is also called within componentDidMount, that method must also be bound to the component instance:

VK.init(function() {
  console.info("API initialisation successful");
  VK.api('users.get', { fields: 'photo_50' }, function(data) {
    if (data.response) {
      this.setState({ // the error happens here
        FirstName: data.response[0].first_name
      });
      console.info(this.state.FirstName);
    }
  }.bind(this));
}.bind(this), function() {
  console.info("API initialisation failed");
}, '5.34');

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