how to debug code with console.log when the code runs slow

Question

I'm practicing react, and just now I'm about to make a to-do list. While i was working. I wanted to test my code with console.log to see input values are passing correctly to state. However for awhile I was confused to see how console.log would always output the previous state. Until later, i just embeded <p>{this.state.myArray}</p> and it shows it is working correctly. I presume this.setState({ myArray: this.state.message }); is still finishing executing while console.log already executed.

I'm pretty sure im using console.log the wrong way to test code. New programmer here.

class ToDoInput extends React.Component {
  constructor(props) {
    super(props);
    this.handleChange = this.handleChange.bind(this);
    this.handleSubmit = this.handleSubmit.bind(this);
    this.state = { message: "", myArray: "" };
  }

  handleChange(e) {
    this.setState({ message: e.target.value });
    console.log("handleChange: " + this.state.message); //testing
  }

  handleSubmit(e) {
    this.setState({ myArray: this.state.message });
    console.log("handleSubmit: " + this.state.myArray); //testing
  }

  render() {
    return (
      <form>
        <input
          type="text"
          value={this.state.content}
          onChange={this.handleChange}
        />
        <input type="button" value="submit" onClick={this.handleSubmit} />
        <p>{this.state.myArray}</p>
      </form>
    );
  }
}

codeSandBox link

State changes are asychronous. Try passing a callback as the second argument to this.setState, the console log in there — evolutionxbox
– evolutionxbox, Commented Jan 31, 2020 at 22:22
As @evolutionxbox suggests, try this.setState({message: e.target.value}, () => {console.log(this.state.message);}); — Bojan Bedrač
– Bojan Bedrač, Commented Jan 31, 2020 at 22:30

Adam · Accepted Answer · 2020-01-31 22:37:39Z

1

The problem you're running into is that setState is asynchronous and that it does not set the state immediately but after a short delay. There are several reasons that React does this, but one of them is that it allows React to group multiple state changes and then rerender your component a single time, instead of re-rendering every time setState is called.

If you want to use the state after setting the state, you can use the second argument of setState like this:

  handleSubmit(e) {
    this.setState({ myArray: this.state.message }, () => {
      console.log("handleSubmit: " + this.state.myArray);
    });
  }

You can read more about setState is asynchronous here. Although that Github issue is probably more in-depth than you're looking for.

In your example above, it might be worth putting the console.log(this.state) within your render function, as that should always have the most up-to-date state.

edited Jan 31, 2020 at 22:37

answered Jan 31, 2020 at 22:32

Adam

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