TypeError: Cannot read property 'values' of undefined in Reactjs

Question

I'm not very proficient with React. I'm working with a dynamic form where users should be able to dynamically add and remove input fields. I'm unable to save inputs into variables dynamically however.

Code:


class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      values: [],
      type:[],
      name: '',
      frequency: '',
    };
  } 

  handleMetric(i, event) {
     let values = [...this.state.values];
     values[i] = event.target.value;
     this.setState({ values });
     console.log("Metrics: ")
     console.log(values) 
  }

  handleThreshold(i, event) {
    let values = [...this.state.values];
    values[i] = event.target.value;
    this.setState({ values });
    console.log("Threshold: ")
     console.log(values) 
  }

  addClick(){
    this.setState(prevState => ({ values: [...prevState.values, '']}))
  }
  
  removeClick(i){
     let values = [...this.state.values];
     values.splice(i,1);
     this.setState({ values });
  }

  createUI(){
    return this.state.values.map((el, i) => 
        <div key={i}>
         <input type="text" value={el||''} onChange={this.handleChange.bind(this, i)} />
         <input type='button' value='remove' onClick={this.removeClick.bind(this, i)}/>
        </div>          
    )
 }
  
  render() {

    return (
    <div> 
       <div className="card">
          <form onSubmit={this.handleSubmit}>
         
              <Form.Item name="Type of metrics" label="Metric" htmlFor="type">
                <Select
                  value={Select}
                  onChange={this.handleMetric}
                  options={metrics}
                />
              </Form.Item>

              <Form.Item name="amount" label="Threshold Score" htmlFor="threshold">
                <Slider marks={marks} name="threshold" onChange={this.handleThreshold} />
              </Form.Item>

            </Form>

            {this.createUI()}  

            <input type='button' value='add more' onClick={this.addClick.bind(this)}/>
            <input type="submit" value="Submit" />

          </form>
        </div>
      </div>
    );
  }
}

export default App

Both handleThreshold and handleMetric functions are not working, and giving the following errors:

Would really appreciate some help in getting the variables stored in the arrays dynamically.

Ross Allen · Accepted Answer · 2021-02-07 01:46:35Z

1

Turn the class methods into class property initializers with arrow functions so this is always the component instance:

  handleMetric = (i, event) => {
     let values = [...this.state.values];
     values[i] = event.target.value;
     this.setState({ values });
     console.log("Metrics: ")
     console.log(values) 
  };

  handleThreshold = (i, event) => {
    let values = [...this.state.values];
    values[i] = event.target.value;
    this.setState({ values });
    console.log("Threshold: ")
     console.log(values) 
  };

You solved this for another callback by doing this.addClick.bind(this) inside the render method. Both approaches work. Doing the arrow function declaration means only 1 function is created in the life of the component whereas .bind in render means 1 function is created per call to render. This will likely have no recognizable performance change for your app but is something to consider.

answered Feb 7, 2021 at 1:46

Ross Allen

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3 Comments

hi_it'sme Over a year ago

Thank you, but it isn't working. I'm getting an error on line values[i] = event.target.value;

Ross Allen Over a year ago

What error are you getting on that line now?

hi_it'sme Over a year ago

I got the same error as before "value not defined", but I've got it working now, thank you anyway!

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