What scopes were introduced in es6

The "block-level scope" was introduced in es6; in es5, there are only global scope and function scope. In es6, the scope formed by a pair of curly brackets is the block-level scope, and it is explicitly allowed to When a function is declared in a block-level scope, the function declaration statement behaves like let and cannot be referenced outside the block-level scope.

The operating environment of this tutorial: Windows 10 system, ECMAScript version 6.0, Dell G3 computer.

What scopes were introduced in es6

ES6 introduced block-level scope, explicitly allowing functions to be declared in block-level scope. ES6 stipulates that in block-level scope, function declaration statements behave like let and cannot be referenced outside block-level scope.

In the ES6 standard, the scope formed by a pair of braces is the block-level scope.

ES5 stipulates that functions can only be declared in the top-level scope and function scope, and cannot be declared in the block-level scope.

ES5 only has global scope and function scope, but no block-level scope, which brings many unreasonable scenarios.

(1) Inner variables may overwrite outer variables.

    var tmp = new Date();
    function f() {
        console.log(tmp);
        if (false) {
            var tmp = 'hello world';
        }
    }
    f();//undefined

(2) The loop variable used for counting is leaked into a global variable

    var s = 'hello';
    for (var i = 0; i < s.length; i++) {
        console.log(s[i]);//h e l l o
    }
    console.log(i);//5

The variable i is only used to control the loop, but after the loop ends, it does not disappear and is leaked into a global variable .

An example is as follows:

// 浏览器的 ES6 环境
function f() { console.log('I am outside!'); }
(function () {
  if (false) {
    // 重复声明一次函数f
    function f() { console.log('I am inside!'); }
  }
  f();
}());
// Uncaught TypeError: f is not a function

In the ES6 standard, when a function is declared in a block-level scope, the function declaration will be regarded as a variable declared by var and will be promoted to the top of the block-level scope. When the function is used as a variable and the variable name is declared, the function body will not be declared. The above code is equivalent to:

// 浏览器的 ES6 环境
function f() { console.log('I am outside!'); }
(function () {
  var f = undefined;
  if (false) {
    function f() { console.log('I am inside!'); }
  }
  f();
}());
// Uncaught TypeError: f is not a function

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