JavaScript for循环

在编程中，循环用于重复代码块。

例如，如果要显示一条消息100次，则可以使用循环。这只是一个简单的例子。使用循环可以实现更多目标。

本教程重点介绍JavaScript for循环。您将在即将到来的教程中了解其他类型的循环。

for循环的语法为：

for(initialExpression; condition; updateExpression) {
    // for loop body
}

这里，

initialExpression初始化和/或声明变量，并且仅执行一次。
条件被评估。
- 如果条件为false ，则for循环终止。
- 如果条件为true ，则执行for循环内的代码块。
当条件是updateExpression更新initialExpression的价值true 。
再次评估条件。此过程将继续，直到条件为false为止。

要了解有关条件的更多信息，请访问JavaScript比较和逻辑运算符。

Working of for loop in JavaScript with flowchart — JavaScript循环流程图

示例1：显示文本五次

// program to display text 10 times
let n = 5;

// looping from i = 1 to 5
for (let i = 1; i <= n; i++) {
    console.log(`I love JavaScript.`);
}

输出

I love JavaScript.
I love JavaScript.
I love JavaScript.
I love JavaScript.
I love JavaScript.

该程序的工作原理如下。

Iteration	Variable	Condition: i <= n	Action
1st	`i = 1` `n = 5`	`true`	`I love JavaScript.` is printed. `i` is increased to 2.
2nd	`i = 2` `n = 5`	`true`	`I love JavaScript.` is printed. `i` is increased to 3.
3rd	`i = 3` `n = 5`	`true`	`I love JavaScript.` is printed. `i` is increased to 4.
4th	`i = 4` `n = 5`	`true`	`I love JavaScript.` is printed. `i` is increased to 5.
5th	`i = 5` `n = 5`	`true`	`I love JavaScript.` is printed. `i` is increased to 6.
6th	`i = 6` `n = 5`	`false`	The loop is terminated.

示例2：显示从1到5的数字

// program to display numbers from 1 to 5
let n = 5;

// looping from i = 1 to 5
// in each iteration, i is increased by 1
for (let i = 1; i <= n; i++) {
    console.log(i);     // printing the value of i
}

输出

该程序的工作原理如下。

Iteration	Variable	Condition: i <= n	Action
1st	`i = 1` `n = 5`	`true`	`1` is printed. `i` is increased to 2.
2nd	`i = 2` `n = 5`	`true`	`2` is printed. `i` is increased to 3.
3rd	`i = 3` `n = 5`	`true`	`3` is printed. `i` is increased to 4.
4th	`i = 4` `n = 5`	`true`	`4` is printed. `i` is increased to 5.
5th	`i = 5` `n = 5`	`true`	`5` is printed. `i` is increased to 6.
6th	`i = 6` `n = 5`	`false`	The loop is terminated.

示例3：显示n个自然数的总和

// program to display the sum of natural numbers
let sum = 0;
let n = 100

// looping from i = 1 to n
// in each iteration, i is increased by 1
for (let i = 1; i <= n; i++) {
    sum += i;  // sum = sum + i
}

console.log('sum:', sum);

输出

sum: 5050

在这里， sum的值最初为0 。然后，将for循环从i = 1 to 100迭代i = 1 to 100 。在每次迭代中，将i加到sum上，并将其值增加1 。

当i变为101时 ，测试条件为false ， 总和等于0 +1 + 2 + … + 100 。

上面的程序加上自然数的和也可以写成

// program to display the sum of n natural numbers
let sum = 0;
let n = 100;

// looping from i = n to 1
// in each iteration, i is decreased by 1
for(let i = n; i >= 1; i-- ) {
    // adding i to sum in each iteration
    sum += i; // sum = sum + i
}

console.log('sum:',sum);

该程序还提供与示例3相同的输出。您可以在编程中以多种不同的方式完成相同的任务。编程全部与逻辑有关。

尽管两种方法都是正确的，但是您应该尝试使代码更具可读性。

JavaScript Infinite for循环

如果for循环中的测试条件始终为true ，则它将永远运行(直到内存已满)。例如，

// infinite for loop
for(let i = 1; i > 0; i++) {
    // block of code
}

在以上程序中，条件始终为true ，然后将无限次运行代码。

在下一个教程中，您将学习while和do...while循环。

在本教程中，您将借助示例来了解JavaScript中的循环和for循环。