Loop

Concept

In JavaScript programming, loops are a powerful tool for performing repetitive tasks. We can use loops to execute a block of code multiple times, check conditions, and manipulate elements in an array.

The for loop

The for loop is one of the most common types of loops in JavaScript. It allows us to execute a block of code multiple times based on a counter variable. The syntax of the for loop is as follows:

for (khởi tạo; điều kiện; bước nhảy) {
  // Khối mã thực thi
}

Here is an example of how to use the for loop to display numbers from 1 to 5:

for (let i = 1; i <= 5; i++) {
  console.log(i);
}

The result will be displayed on the console as follows:

1
2
3
4
5

The while loop

The while loop allows us to execute a block of code repeatedly while a specific condition is true. The syntax of the while loop is as follows:

while (điều kiện) {
  // Khối mã thực thi
}

Here is an example of how to use the while loop to calculate the sum of numbers from 1 to 5:

let sum = 0;
let i = 1;

while (i <= 5) {
  sum += i;
  i++;
}

console.log(sum); // Kết quả: 15

The do...while loop

The do...while loop works similar to the while loop, but the code block is executed at least once before checking the condition. The syntax of the do...while loop is as follows:

do {
  // Khối mã thực thi
} while (điều kiện);

Here is an example of how to use the do...while loop to display numbers from 1 to 5:

let i = 1;

do {
  console.log(i);
  i++;
} while (i <= 5);

The result will be displayed on the console as follows:

1
2
3
4
5

The for...in loop

The for...in loop is used to iterate over the properties of an object. The syntax of the for...in loop is as follows:

for (biến in đối tượng) {
  // Khối mã thực thi
}

Here is an example of how to use the for...in loop to iterate over the properties of an object:

let person = {
  name: "John",
  age: 30,
  city: "New York"
};

for (let key in person) {
  console.log(key + ": " + person[key]);
}

The result will be displayed on the console as follows:

name: John
age: 30
city: New York

The for...of loop

The for...of loop is used to iterate over the elements of an iterable object such as an array. The syntax of the for...of loop is as follows:

for (biến of đối tượng) {
  // Khối mã thực thi
}

Here is an example of how to use the for...of loop to iterate over elements in an array:

let numbers = [1, 2, 3, 4, 5];

for (let number of numbers) {
  console.log(number);
}

The result will be displayed on the console as follows:

1
2
3
4
5

forEach loop

The forEach loop allows us to access each element in the array and perform a specific action with each element. We can use a callback function passed to the forEach method to perform that action. The syntax of forEach is as follows:

array.forEach(function(giá trị hiện tại, vị trí trong mảng, mảng) {
  // Khối mã thực thi
});

Here is an example of how to use the forEach loop to iterate over elements in an array:

let numbers = [1, 2, 3, 4, 5];

numbers.forEach(function(number) {
  console.log(number);
});

The result will be displayed on the console as follows:

1
2
3
4
5