javascript es6: classic interview questions (var and let)

where:

	var arr = [];

		for (var i = 0; i < 2; i++) {
			arr[i] = function () {
				console.log(i);
			}
		}
		arr[0]();
		arr[1]();

---

Output:

2
2

---

Analysis:

The function inside the for loop is never executed.

When the function is executed, it cannot find its own value of variable i in its own scope. According to the scope chain search principle, it is necessary to search in the upper layer scope (that is, the global scope). There is a variable i in the global scope, because When the function is executed, the loop has already been executed. When i = 2, the loop is completed, the printed i is a global variable, so the output of both functions isGlobal variable iValue of 2

The key point of this problem is that the variable i is global, and the output of the function when the function is executed is the value of i under the global action.

---

let：

 let arr = [];

        for (let i = 0; i < 2; i++) {
            arr[i] = function() {
                console.log(i);
            }
        }
        arr[0]();
        arr[1]();

---

Output:

0
1

---

analysis:

Since the variable i declared with let has block-level scope, two block-level scopes are generated after the end of the loop.

The two block-level scopes generated have their own variables i. These are two variables, which do not affect each other, because in different block-level scopes, two variables i are still stored in the array after the loop is over. Function During execution, there is still no own variable i, and we still have to look up the upper-level scope. In the current code, the upper-level scope of the function is actually the block-level scope generated by the loop, so the two array functions need to be executed. Find the value of i in the corresponding block-level scope.

The key point of this problem is that each loop will generate a block-level scope. The variables in each block-level scope are different. When the function is executed, it outputs its own upper level (block-level scope generated by the loop) Under value