I don't know what you understood before.Now you can judge the output of the following code

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  btn.addEventListener('click', function(){ console.log(i); });
  document.body.appendChild(btn);
}

Familiar with that, right, you can do what you want with a few changes

for (var i = 0; i < 5; i++) {
  var btn = document.createElement('button');
  btn.appendChild(document.createTextNode('Button ' + i));
  (function(i){
  	btn.addEventListener('click', function(){ console.log(i); });
  })(i);
  document.body.appendChild(btn);
}

You can try the console yourself to see if it works.

So let's judge the following code

var arr1 = "john".split(''); j o h n
var arr2 = arr1.reverse(); n h o j
var arr3 = "jones".split(''); j o n e s
arr2.push(arr3);
console.log("array 1: length=" + arr1.length + " last=" + arr1.slice(-1));
console.log("array 2: length=" + arr2.length + " last=" + arr2.slice(-1));

There are two main points. The first is that the reverse() method changes the original array, that is, arr1 becomes n h o j

Another important point is that similar to the form a = [0,1,2,3], b = a, changing the value of b will change the value of a, but if b is reassigned, the value of b will not change

var a = [10,11,12]
var b = a;
b.push(55);
console.log(b);
console.log(a);

So what if this is the case?

var a = [10,11,12]
var b = a;
b = [1,2,5,8]
console.log(b);
console.log(a);

Running the answer is obvious

demo thirteen

Direct Up Code

console.log(1 +  "2" + "2");
console.log(1 +  +"2" + "2");
console.log(1 +  -"1" + "2");
console.log(+"1" +  "1" + "2");
console.log( "A" - "B" + "2");
console.log( "A" - "B" + 2);

Remember a few rules

1. If a number string is preceded by a'+','-', it will be converted to a number during the operation.

2. Returns NaN for strings that cannot be evaluated