How to explain this phenomenon in JS? What is the principle?

data obtains the data asynchronously, then saves it in the local variable after processing.
the first console shows a value
but the second cannot get the index
the third can get the index after 400 milliseconds. Why?

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Javascript
May.22,2021
The reason for this is actually that the data you see on the console will be changed by subsequent operations because the array is a reference type , so what you see on the console is not what it printed out at that time
you can try this code 

const data = [];

setTimeout(() => {
  data.push({})
}, 100);

console.log('1', data);
console.log('2', JSON.stringify(data));
console.log('3', data[0]);

setTimeout(() => {
  console.log('4', data[0]);
}, 400);

whether it is correct to understand that 1 and 2 should be the same, but the reality is

Asynchronous didn't wait, right? 400 milliseconds when the asynchronous request ends

this.data and the this.data, in the timer point to the same?

my understanding is that it takes time to traverse data. When console, it does not traverse data [0], so it is undefined

.

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