IIFE是JavaScript中定義后立即執(zhí)行的函數(shù)表達式，用于創(chuàng)建私有作用域并避免全局變量污染。其基本結(jié)構(gòu)是將函數(shù)包裹在括號內(nèi)并緊隨一對括號調(diào)用它，如(function() { // code })()，也可使用箭頭函數(shù)；外層括號使函數(shù)被視為表達式而非聲明，最后的括號執(zhí)行該函數(shù)。1. 使用IIFE可避免全局變量沖突，防止與其他腳本產(chǎn)生命名沖突；2. 創(chuàng)建私有作用域，使內(nèi)部變量和函數(shù)無法被外部訪問；3. 在ES6模塊出現(xiàn)前，IIFE常用于模擬模塊模式，返回公開方法同時隱藏內(nèi)部邏輯。盡管現(xiàn)代JS已有l(wèi)et、const和模塊減少IIFE需求，但在舊項目、HTML內(nèi)聯(lián)腳本、快速測試或一次性代碼塊中仍適用。需要注意正確閉合括號、避免表達式干擾行為、以及在嚴格模式下默認增強代碼規(guī)范性。

IIFEs, or Immediately Invoked Function Expressions, are functions in JavaScript that are executed right after they are defined. They’re commonly used to create a private scope and avoid polluting the global namespace.

What Exactly Is an IIFE?

An IIFE is a function that runs as soon as it’s defined. It wraps a function in parentheses and adds another set of parentheses at the end to call it immediately.

Here's the basic structure:

(function() {
  // code here
})();

You can also use arrow functions (though less common for IIFEs):

(() => {
  // code here
})();

The outer parentheses tell JavaScript to treat the function as an expression, not a declaration. The final () actually invokes the function.

Why Use an IIFE?

There are a few solid reasons why developers still reach for IIFEs, especially in certain scenarios:

• Avoid Global Variable Conflicts
  Variables declared inside an IIFE aren’t exposed globally. That means you won’t accidentally overwrite variables from other scripts.

• Create Private Scope
  If you need temporary variables or functions that shouldn’t be accessible outside a specific context, an IIFE gives you a clean way to do that.

• Module Pattern Foundation
  Before ES6 modules, IIFEs were often used to simulate module-like behavior—returning an object with public methods while keeping some logic private.

For example:

const counter = (function() {
  let count = 0;

  return {
    increment: () =>   count,
    decrement: () => --count,
    get: () => count
  };
})();

Now counter has access to increment, decrement, and get, but count stays private.

When Should You Still Use IIFEs?

Even though modern JavaScript has let, const, and modules that reduce the need for IIFEs, there are still cases where they come in handy:

• In older projects or legacy codebases that don’t use ES6
• When creating a self-contained block of code inside a script tag (e.g., in HTML inline scripts)
• For quick tests or sandboxed environments during development

Also, if you're writing code that needs to run once and doesn’t need to be reused or exported, an IIFE can be a clean approach.

A Few Things to Watch Out For

• Don’t forget the closing })(); — it's easy to miss a parenthesis and break everything.
• Make sure your IIFE isn’t accidentally part of an expression that changes its behavior.
• In strict mode ("use strict"), IIFEs help enforce better coding practices by default.

If you're using modern JavaScript features like modules and top-level await, you might not need IIFEs very often anymore — but knowing how they work is still useful.

基本上就這些。

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