在内联函数中使用时，为什么会出现“ no-unused-var”错误

Question

Here's a snippent of my code:

const prevState = this.state;
this.setState(prevState => ({
  companies_checked: {
    ...prevState.companies_checked,
    [target.value]: target.checked
  }
}));

It is generating the following error:

Line 52:  'prevState' is assigned a value but 
never used  no-unused-vars

I'm using the variable in the inline/arrow function. What's the best way to make the error go away?

Answer 1

I'm using the variable in the inline/arrow function.

No, you don't. The prevState that you use in the function is declared as the function's parameter. It is shadowing the const -declared prevState from the outer scope.

What's the best way to make the error go away?

Assuming the function is working as is, just omit the const declaration.

Answer 2

Try this (ie remove the line where you declare prevState ):

this.setState(prevState => ({
  companies_checked: {
    ...prevState.companies_checked,
    [target.value]: target.checked
  }
}));

Eslint is complaining that you have declared the constant prevState without ever using it, afterwards. The correct way to fix the problem is to simply not declare it in the first place.

The prevState inside your setState call is correct - the first argument setState sends to your callback function is the previous state, and this is the one you want to use inside the callback function.

Answer 3

I think it is just naming conflict.

  // you already declared prevState
    const prevState = this.state;
    // change to this.setState(previousState => 
    this.setState(prevState => ({
      companies_checked: {
        ...prevState.companies_checked,
        [target.value]: target.checked
      }
    }));

You actually did not use prevState that you declared