js에서 let과 var 정의 변수의 차이점은 무엇입니까?

let hello = 'hello world.';
console.log(hello);

let hello = 'hello world.';
^^^
SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
 ...

'use strict';
let hello = 'hello world.';
console.log(hello);

'use strict';
(function() {
 var varTest;
 let letTest;
 console.log(varTest); //输出undefined
 console.log(letTest); //输出undefined
}());

(function() {
 console.log(varTest); //输出undefined(注意要注释掉下面一行才能运行)
 console.log(letTest); //直接报错：ReferenceError: letTest is not defined
 var varTest = 'test var OK.';
 let letTest = 'test let OK.';
}());

'use strict';
(function() {
 var varTest = 'test var OK.';
 let letTest = 'test let OK.';
 var varTest = 'varTest changed.';
 let letTest = 'letTest changed.'; //直接报错：SyntaxError: Identifier 'letTest' has already been declared
 console.log(varTest); //输出varTest changed.(注意要注释掉上面letTest变量的重复声明才能运行)
 console.log(letTest);
}());