如果我使用 var 而不是 let in for 循环，为什么代码返回 undefined

Question

Why does my code return undefined if I use var inside a for loop instead of let. I have attached the output for let and var below the code blocks.

const a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

for (let i = 0; i < 10; i++) {
  setTimeout(() => console.log(a[i]), 3000);
}

Output : [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

var i = 0;
for (i = 0; i < 10; i++) {
  setTimeout(() => console.log(a[i]), 3000);
}

// Output : [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined ]

Answer 1

Variables declared by var keyword are scoped to the immediate function body so its falls under the function scope. And let variables are scoped to the immediate enclosing block denoted by { } - the block scope.