在 JavaScript 中的循環內建立函數時如何避免閉包問題？

JavaScript Closure Inside Loops - A Practical Example

When creating functions within loops in JavaScript, it's essential to be aware of the potential for closure issues where the variables used in those functions can be mistakenly bound to the same value. This can lead to unexpected behavior, particularly when delays are involved.

Problem Statement

Consider the following code:

<code class="js">const funcs = [];
for (var i = 0; i < 3; i++) {
  funcs[i] = function() {
    console.log("My value:", i);
  };
}</code>

The intended output is:

<code>My value: 0
My value: 1
My value: 2</code>

However, this code actually outputs:

<code>My value: 3
My value: 3
My value: 3</code>

This problem occurs because the variable i within the anonymous functions is bound to the same variable outside of the loop. As a result, when each function is executed, it uses the final value of i.

ES6 Solution: let

In ECMAScript 6 (ES6), the let keyword allows for block-scoped variables. Using let within loops creates a new variable with each iteration, resolving the closure issue.

<code class="js">for (let i = 0; i < 3; i++) {
  funcs[i] = function() {
    console.log("My value: " + i);
  };
}</code>

ES5.1 Solution: forEach

For situations where you're primarily iterating over an array, the Array.prototype.forEach function can provide a clean solution.

<code class="js">someArray.forEach(function(arrayElement) {
  // ... code for this one element
});</code>

Each invocation of the callback function will be its own closure, ensuring that the parameter passed in is specific to that iteration.

Classic Solution: Closures

Another method to avoid closure issues is to use classic closures, which involve binding the variable within each function to a separate, unchanging value.

<code class="js">function createfunc(i) {
  return function() {
    console.log("My value: " + i);
  };
}</code>

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