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js中函數(shù)聲明與函數(shù)表達(dá)式的區(qū)別以及立即執(zhí)行函數(shù)

madthumb / 2914人閱讀

摘要：最近在寫(xiě)代碼時(shí)遇到了閉包，其中閉包又和立即執(zhí)行函數(shù)有點(diǎn)關(guān)系，于是牽扯除了函數(shù)聲明以及函數(shù)表達(dá)式，我感覺(jué)中文的很多文章寫(xiě)的不太好，查閱了的指南和這篇關(guān)于的文章，覺(jué)得寫(xiě)的很好，整合一下。函數(shù)聲明和函數(shù)表達(dá)式。

最近在寫(xiě)代碼時(shí)遇到了閉包，其中閉包又和立即執(zhí)行函數(shù)（immediately invoked function expression, aka IIFE)有點(diǎn)關(guān)系，于是牽扯除了函數(shù)聲明以及函數(shù)表達(dá)式，我感覺(jué)中文的很多文章寫(xiě)的不太好，查閱了mdn的function指南和這篇關(guān)于IIFE的文章，覺(jué)得寫(xiě)的很好，整合一下。

Firstly, in JS there are two ways of defining functions: function declarations and function expressions, aka 函數(shù)聲明和函數(shù)表達(dá)式。

function declaration (function definition or function statement)

This is always followed by the name of the function (which is very important cauz function declaration requires a name, if not, it may encounter some SyntaxError, we can see from the following examples), a list of parameters and the JavaScript statements that define the function, enclosed in curly braces{}.

function expression

This can be anonymous; it doesn"t have to have a name;

var ab = function(){}; //anonymous function expression
var ac = function ad() {};// function expression with a name

.
.
SyntaxError will happen in the following case when lack of a name:

var foo = function(){}; //this is a anonymous function expression we will see in the future
foo(); //we can invoke the function by adding parens after foo
// then some people might think of the following, since foo equals to function(){} and foo() can be used, why don"t we use function{}()?
function(){}(); // However, there will be a SyntaxError: "unexpected ("

1.1 What leads to this SyntaxError?
The reason is that the parser treats it as a function declaration without a name rather than a function expression.
Now that, what about we give it a name by using

function foo(){}();// there will also be a SyntaxError: "unexpected )"

1.2 What leads to the other SyntaxError?
This is because parens placed after the stataments are view as a grouping operator which needs to contain an expression;
OK, here I finally got the idea and there must be no error! Why don"t we use this:

function foo(){}(1); //no error

1.3 In this case, however, the function is not executed either despite there is no error. Here is the reason, the parser treated this as two separate unrelated expression

function foo(){}; 
(1);

.
.
Is there any way to solve the problem????
Yep! The solution is IIFE（立即執(zhí)行函數(shù)）?。。。。。?/p> 

(function(){})();
(function(){}());

Unlike in 1.1, the parser encounters function keyword and knows that now I need to treat it like function expression rather than a function declaration! Such parens indicate that the function expression will be immediately invoked and the variable will contain the result of the function.
也就是說(shuō)，只要不讓function這個(gè)關(guān)鍵詞出現(xiàn)在行首就行，所以下面的情況都可以。

~function(){}();
!function(){}();
One application of IIFE -- Closure(閉包）

IIFE is used to lockin values and effectively save state.
The advantage is that since the function is invoked immediately without an identifier, a closure can be used without polluting the current scope.

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