使用“let”和“let”之间的区别是什么？和“var”在JavaScript中声明变量？

答

区别在于范围界定。 var 的范围限定为最近的功能块，将限定为最近的封闭块，它可以小于功能块。如果在任何区域之外，两者都是全局的。

The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block, which can be smaller than a function block. Both are global if outside any block.

此外，用 let 声明的变量在声明之前是不可访问的它们的封闭块。如演示中所示，这将引发ReferenceError异常。

Also, variables declared with let are not accessible before they are declared in their enclosing block. As seen in the demo, this will throw a ReferenceError exception.

演示：

Demo:

var html = '';

write('#### global ####\n');
write('globalVar: ' + globalVar); //undefined, but visible

try {
  write('globalLet: ' + globalLet); //undefined, *not* visible
} catch (exception) {
  write('globalLet: exception');
}

write('\nset variables');

var globalVar = 'globalVar';
let globalLet = 'globalLet';

write('\nglobalVar: ' + globalVar);
write('globalLet: ' + globalLet);

function functionScoped() {
  write('\n#### function ####');
  write('\nfunctionVar: ' + functionVar); //undefined, but visible

  try {
    write('functionLet: ' + functionLet); //undefined, *not* visible
  } catch (exception) {
    write('functionLet: exception');
  }

  write('\nset variables');

  var functionVar = 'functionVar';
  let functionLet = 'functionLet';

  write('\nfunctionVar: ' + functionVar);
  write('functionLet: ' + functionLet);
}

function blockScoped() {
  write('\n#### block ####');
  write('\nblockVar: ' + blockVar); //undefined, but visible

  try {
    write('blockLet: ' + blockLet); //undefined, *not* visible
  } catch (exception) {
    write('blockLet: exception');
  }

  for (var blockVar = 'blockVar', blockIndex = 0; blockIndex < 1; blockIndex++) {
    write('\nblockVar: ' + blockVar); // visible here and whole function
  };

  for (let blockLet = 'blockLet', letIndex = 0; letIndex < 1; letIndex++) {
    write('blockLet: ' + blockLet); // visible only here
  };

  write('\nblockVar: ' + blockVar);

  try {
    write('blockLet: ' + blockLet); //undefined, *not* visible
  } catch (exception) {
    write('blockLet: exception');
  }
}

function write(line) {
  html += (line ? line : '') + '<br />';
}

functionScoped();
blockScoped();

document.getElementById('results').innerHTML = html;

<pre id="results"></pre>

在功能块之外使用它们非常相似。

They are very similar when used like this outside a function block.

let me = 'go';  // globally scoped
var i = 'able'; // globally scoped

但是，用定义的全局变量让将不会作为属性添加到全局窗口对象，例如使用 var 定义的对象。

However, global variables defined with let will not be added as properties on the global window object like those defined with var.

console.log(window.me); // undefined
console.log(window.i); // 'able'

功能：

在功能块中使用时它们是相同的。

Function:

They are identical when used like this in a function block.

function ingWithinEstablishedParameters() {
    let terOfRecommendation = 'awesome worker!'; //function block scoped
    var sityCheerleading = 'go!'; //function block scoped
}

阻止：

这是区别。 让仅在 for（）循环和 var 中可见>对整个函数可见。

Block:

Here is the difference. let is only visible in the for() loop and var is visible to the whole function.

function allyIlliterate() {
    //tuce is *not* visible out here

    for( let tuce = 0; tuce < 5; tuce++ ) {
        //tuce is only visible in here (and in the for() parentheses)
        //and there is a separate tuce variable for each iteration of the loop
    }

    //tuce is *not* visible out here
}

function byE40() {
    //nish *is* visible out here

    for( var nish = 0; nish < 5; nish++ ) {
        //nish is visible to the whole function
    }

    //nish *is* visible out here
}

重新声明：

假设严格模式， var 将允许您在同一范围内重新声明相同的变量。另一方面，让不会：

Redeclaration:

Assuming strict mode, var will let you re-declare the same variable in the same scope. On the other hand, let will not:

'use strict';
let me = 'foo';
let me = 'bar'; // SyntaxError: Identifier 'me' has already been declared

'use strict';
var me = 'foo';
var me = 'bar'; // No problem, `me` is replaced.