IIFE内部的可变提升(延迟解析) [英] Variable hoisting inside IIFE (lazy parsing)

问题描述

在以下情况下，我得到一个非常奇怪的输出:

I am getting a very strange output on the below scenarios:

function test(){
   var test=123;
   console.log(test)
}
// this output : 123

(function test(){
   var test=123;
   console.log(test)
})()

// this output: 123 

但是使用以下代码时

(function test(){
   test=123;
   console.log(test)
})()

//output:

function test(){
    test=123;
    console.log(test)
}

任何人都可以解释一下.

Can anybody please explain.

推荐答案

您看到的与吊装无关.

您的第一个示例非常简单:

Your first example is quite straightforward:

(function test(){
   var test=123;
   console.log(test)
})()

您正在函数范围内创建一个名为 test 的变量(通过 var )，并为其分配一个值，然后输出该值.

You're creating a variable (via var) called test within the scope of the function, and assigning it a value, then outputting that value.

您的第二个示例省略了 var :

Your second example leaves out var:

(function test() {
    test = 123;
    console.log(test);
})();

...因此 test 的含义完全不同:在由命名函数表达式创建的函数中，函数的名称是解析该函数​​的标识符.因此，该代码中的 test 是该函数的标识符.

...and so test has quite a different meaning: Within a function created by a named function expression, the function's name is an identifier resolving to the function. So test in that code is an identifier for the function.

使用函数表达式时，该标识符为只读，因此您的 test = 123; 行将被忽略，而 console.log行输出该函数的表示形式(好像根本没有 test = 123; 一样.)

That identifier is read-only when you've used a function expression, so your test = 123; line is ignored, and the console.log line outputs a representation of the function (as though test = 123; weren't there at all).

我忘了标识符是只读的(由表达式创建时)，但是它是:来自

I'd forgotten that identifier is read-only (when created by an expression), but it is: From the specification:

FunctionExpression:functionBindingIdentifier(FormalParameters){FunctionBody}

1. 如果 FunctionExpression 的功能代码是严格模式代码，则将 strict 设置为true.否则，将 strict 设置为false.
2. 让 scope 成为正在运行的执行上下文的LexicalEnvironment.
3. 让 funcEnv 为NewDeclarativeEnvironment(范围).
4. 让 envRec 成为 funcEnv 的EnvironmentRecord.
5. 让 name 为 BindingIdentifier 的StringValue.
6. 执行envRec.CreateImmutableBinding( name ，false).
7. ...

1. If the function code for FunctionExpression is strict mode code, let strict be true. Otherwise let strict be false.
2. Let scope be the running execution context's LexicalEnvironment.
3. Let funcEnv be NewDeclarativeEnvironment(scope).
4. Let envRec be funcEnv's EnvironmentRecord.
5. Let name be StringValue of BindingIdentifier.
6. Perform envRec.CreateImmutableBinding(name, false).
7. ...

注意步骤6:创建标识符的绑定是不可变(无法更改).

Note Step 6: The binding creating the identifier is immutable (can't be changed).

请注意，对于可变的函数声明创建的标识符(绑定)而言，这不是正确的；但是函数 expressions 和函数 declarations 对由函数名称创建的标识符的处理完全不同.(例如:函数声明将名称放在声明所在的作用域中，而函数表达式则不在.)

Note that this isn't true for the identifier (binding) created by a function declaration, which is mutable; but function expressions and function declarations treat the identifiers created by the function name completely differently. (For instance: A function declaration puts the name in scope where the declaration is, but a function expression doesn't.)

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