React Beginner Question: Confusing Arrow Function Example

Question

I'm a React/ES6 beginner and I'm using this code I found to handle a checkbox inside a custom component being clicked (the custom component includes a material-ui CheckBox, hence the "checked" value). I'm planning on adding more fields to the custom component, such as a textbox that corresponds to the checkbox where the user can add more information about the box they checked.

Anyway, I'm a bit confused about what's going on in that first line. I was hoping one of you senior level devs could break it down for me so i can understand what's happening here.

Two things to note:

1. index console logs as an integer value (position in my mapped array)

2. checked is false by default but console logs as true (is it being toggled true somehow?)

   const onMediaDeliverableChange = index => ({ target: {checked} }) => {
   
   console.log('>> [form.js] (onMediaDeliverablesChange) index =  ',index);
   console.log('>> [form.js] (onMediaDeliverablesChange) target =  ',checked); }
   

Here's an example of code that I took this from, that is working.

const onCheckBoxChange = index => ({ target: { checked } }) => {
    const newValues = [...values];
    const value = values[index];
    newValues[index] = { ...value, checked };

    console.log('>> [form.js] (onCheckBoxChange) value = ',value, index);
    console.log('>> [form.js] (onCheckBoxChange) newValues[index] = ',newValues[index]);

    props.setDesignOrDigital(newValues);

  };

Answer 1

The following:

const onCheckBoxChange = index => ({ target: { checked } }) => {
  // stuff
};

could also be written as:

const onCheckBoxChange = index => (event) => {
  const checked = event.target.checked;
  // stuff
};

{ target: { checked } } is an example of using object destructuring on a function argument. In this case, the argument would be an event.

The first part:

index => another-function

is, as svey mentioned in the comments, an example of a curried function. I assume onCheckBoxChange was used in a loop where the index of the checkbox being rendered was passed in which then returns a function that is an event handler for the change event. The index is then used to get/set the checked state for the respective checkboxes.