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2620. Counter

Given an integer n, return a counter function. This counter function initially returns n and then returns 1 more than the previous value every subsequent time it is called (n, n + 1, n + 2, etc).

Example 1:

Input: 
n = 10 
["call","call","call"]
Output: [10,11,12]
Explanation: 
counter() = 10 // The first time counter() is called, it returns n.
counter() = 11 // Returns 1 more than the previous time.
counter() = 12 // Returns 1 more than the previous time.

Example 2:

Input: 
n = -2
["call","call","call","call","call"]
Output: [-2,-1,0,1,2]
Explanation: counter() initially returns -2. Then increases after each sebsequent call.

Constraints:

  • -1000 <= n <= 1000
  • 0 <= calls.length <= 1000
  • calls[i] === "call"

Solution:

function createCounter(n: number): () => number {
    return function() {
        return n++;
    }
}


/** 
 * const counter = createCounter(10)
 * counter() // 10
 * counter() // 11
 * counter() // 12
 */

Closure（闭包)

  • n 是 createCounter 的局部变量
  • 内层函数使用了 n，JS 会将它捕获，形成 closure
  • 即使 createCounter 执行结束，n 也不会被销毁
const counter = createCounter(10);
counter(); // 10
counter(); // 11
counter(); // 12
  • n++ 能记住状态，是因为闭包保存了私有变量 n
  • 每次 createCounter 调用都会生成独立的闭包环境
const c1 = createCounter(5);
const c2 = createCounter(100);
  • c1 和 c2 互不影响

❌ 如果 n 是全局变量，所有 counter 会共享状态（不安全） ✅ 闭包 = 函数 + 外部变量环境（私有状态）

Js 特性

Java 中没有闭包, 用class使用