在 JavaScript 循環中使用閉包時，如何確保每個閉包捕獲正確的值？

Closures in JavaScript Loops: A Practical Example

In JavaScript, a common issue arises when using closures within loops, where the value of a variable captured by the closure is not the expected value.

Problem:

When defining functions within a loop using the var keyword, all functions reference the same variable outside the loop. This can lead to unexpected results, as seen in the code below:

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

Running this code prints "My value: 3" three times, even though we expect it to increment from 0 to 2.

Solutions:

ES6 Solution: Let

In ECMAScript 6 (ES6), the let keyword introduces block scope for variables, which solves this issue. Using let, each iteration of the loop creates a new variable i with a scope limited to the loop:

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

ES5.1 Solution: forEach

In environments that support the Array.prototype.forEach function, we can leverage its unique nature to create distinct closures for each iteration. The callback function receives the current element of the array as a parameter, ensuring that each function captures a unique value:

<code class="js">var funcs = [];
[0, 1, 2].forEach(function(i) {
  funcs[i] = function() {
    console.log("My value:", i);
  };
});</code>

Classic Solution: Closures

The classic solution is to create a closure that binds the variable to a specific value outside the loop. This is achieved by returning a new function that captures the desired value:

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

By passing i as an argument to the inner function, we ensure that each closure captures the correct value.

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