Answer a question

What is the better way to update state in this context inside a reducer?

case DELETE_INTEREST:
    let deleteInterests = state.user.interests;
    let index = deleteInterests.findIndex(i => i == action.payload);
    deleteInterests.splice(index, 1);
    return { ...state, user: { ...state.user, interests: deleteInterests } };

ESLint doesn't like let statements inside case blocks inside a reducer, getting:

eslint: no-case-declaration - unexpected lexical declaration in case block

Answers

ESLint doesn't like let statements inside case blocks inside a reducer, Why?

This is discouraged because it results in the variable being in scope outside of your current case. By using a block you limit the scope of the variable to that block.

Use {} to create the block scope with case, like this:

case DELETE_INTEREST: {
    let .....
    return (...)
}

Check this snippet:

function withOutBraces() { 
  switch(1){
    case 1: 
      let a=10; 
      console.log('case 1', a); 
    case 2: 
      console.log('case 2', a)
  } 
}

function withBraces() { 
  switch(1){
    case 1: {
      let a=10; 
      console.log('case 1', a); 
    }
    case 2: {
      console.log('case 2', a)
    }
  } 
}

console.log('========First Case ============')
withOutBraces()
console.log('========Second Case ============')
withBraces();

For deleting the element from array, use array.filter, because splice will do the changes in original array. Write it like this:

case DELETE_INTEREST:
    let deleteInterests = state.user.interests;
    let newData = deleteInterests.filter(i => i !== action.payload);
    return { ...state, user: { ...state.user, interests: newData } };