I have these two questions concerning JS. I must say I've really done my research but couldn't find anything about them.
The questions are:

1. Why can't console.log and variable identifiers(var, let, const) be used inside a class, outside of functions/methods?
   E.g.(pseudo code):

class App extends Component {
    console.log(this) <--* ['Unexpected keyword or identifier']
    let a = 1         <--* ['Unexpected token']
    state = {         <--* [variable without identifier is alright]
        characters: [
            {
                name: 'Charlie',
                job: 'Janitor'
            }, etc...
            
    }

    removeCharacter = (index) => {
        console.log(this)  <--* [works here normally]

        const characters = this.state.characters

        etc...
            })
        })
    ...
}

2. Why does a function needs another function to be called? I mean, in what situations does it become necessary? This comes from something like:

const TableBody = (props) => {
    const rows = props.characterData.map((row, index) => {
        return (
            <tr key={index}>
                <td>{row.name}</td>
                <td>{row.job}</td>
 [this] *-->    <button onClick={() => props.removeCharacter(index)}>Delete</button>
            </tr>
        )
 [instead of something like] *--> <button onClick={props.removeCharacter(index)}>Delete</button>

I mean, props.removeCharacter(index) is itself a call already, isn't it?

Thank you!

CodePudding user response：

You can do

class TheClass {
  theProperty = 'theValue'

as part of class field syntax, which is syntax sugar for

class TheClass {
  constructor() {
    this.theProperty = 'theValue'

It's only for assigning properties to the instance, and nothing else.

console.log(this) doesn't work because it doesn't make sense there - it's not assigning a property, it's just a floating expression.

let a = 1 doesn't work because you can't declare a new variable there - the { of a class doesn't create a new block. Using class fields assigns to properties of the instance, and doesn't create new standalone identifiers.

Why does a function needs another function to be called?

In this situation, it's needed because you need to pass an argument. If removeCharacter didn't accept an argument, you could do

<button onClick={props.removeCharacter}>

But it does accept an argument, which is expected to be the index. But the default argument that gets passed to a click handler is the event.

You can't do

onClick={props.removeCharacter(index)}

because that would invoke removeCharacter immediately. It'd be equivalent to

const result = props.removeCharacter(index);
// ...
<button onClick={result}

which isn't the logic that's needed - you want removeCharacter to be called when the button is clicked, not when the button is created.