Are JavaScript event loop operations on variables blocking?

In the non-blocking event loop of JavaScript, is it safe to read and then alter a variable? What happens if two processes want to change a variable nearly at the same time?

Example A:

• Process 1: Get variable A (it is 100)
• Process 2: Get variable A (it is 100)
• Process 1: Add 1 (it is 101)
• Process 2: Add 1 (it is 101)
• Result: Variable A is 101 instead of 102

Here is a simplified example, having an Express route. Lets say the route gets called 1000 per second:

let counter = 0;

const getCounter = () => {
  return counter;
};

const setCounter = (newValue) => {
  counter = newValue;
};

app.get('/counter', (req, res) => {
  const currentValue = getCounter();
  const newValue     = currentValue + 1;
  setCounter(newValue);
});

Example B:

What if we do something more complex like Array.findIndex() and then Array.splice()? Could it be that the found index has become outdated because another event-process already altered the array?

• Process A findIndex (it is 12000)
• Process B findIndex (it is 34000)
• Process A splice index 12000
• Process B splice index 34000
• Result: Process B removed the wrong index, should have removed 33999 instead

const veryLargeArray = [
  // ...
];

app.get('/remove', (req, res) => {
  const id = req.query.id;
  const i  = veryLargeArray.findIndex(val => val.id === id);
  veryLargeArray.splice(i, 1);
});

Example C:

What if we add an async operation into Example B?

const veryLargeArray = [
  // ...
];

app.get('/remove', (req, res) => {
  const id = req.query.id;
  const i  = veryLargeArray.findIndex(val => val.id === id);

  someAsyncFunction().then(() => {
    veryLargeArray.splice(i, 1);
  });
});

This question was kind of hard to find the right words to describe it. Please feel free to update the title.