How to loop through a list in JavaScript

In JavaScript, we often need to loop through lists. This process is very common and is an essential part of programming. This article will introduce how to loop a list in JavaScript so that readers can better master this important skill.

1. for loop

The for loop is one of the most commonly used looping methods in JavaScript. It is suitable for situations where the number of loops is known, such as looping through an array.

The following is an example of using a for loop to traverse an array:

let arr = ['apple', 'banana', 'pear', 'orange'];
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

In the above code, we define an array arr, and then use a for loop to traverse the array and print it out each element.

2. forEach method

In addition to using the for loop to traverse the array, you can also use the forEach() method provided by JavaScript to implement loop operations.

The following is an example of using the forEach() method to traverse an array:

let arr = ['apple', 'banana', 'pear', 'orange'];
arr.forEach(function(item) {
  console.log(item);
});

In the above code, we define an array arr, Then use the forEach() method to iterate through the array and print out each element. forEach()The method needs to pass in a callback function, which will be executed once when each element of the array is traversed.

3. for...in loop

In addition to looping through arrays, there are also situations where JavaScript objects need to be traversed. At this time, we can use for...in loop to implement the traversal operation.

The following is an example of using for...in to loop through objects:

let obj = {
  name: 'Tom',
  age: 20,
  gender: 'male'
};
for (let key in obj) {
  console.log(key + ': ' + obj[key]);
}

In the above code, we define a JavaScript objectobj, then use for...in to loop through the object and print out the key-value pair for each attribute.

Note that the for...in loop does not guarantee the traversal order of object properties, so you need to pay attention to this in practical applications.

4. While loop

The while loop in JavaScript can repeatedly execute a piece of code if the conditions are met. Compared with the for loop, the while loop is more flexible and suitable for situations where the number of loops is uncertain.

The following is an example of using a while loop to print numbers:

let i = 0;
while (i < 10) {
  console.log(i);
  i++;
}

In the above code, we first define the variable i, and then use whileThe loop repeatedly prints out the value of i until the value of i is 10 and breaks out of the loop.

5. do...while loop

do...while loop is similar to while loop, but do.. .whileThe loop will first execute the loop body once, and then determine whether it needs to continue execution based on the conditions.

The following is an example of using a do...while loop to print numbers:

let i = 0;
do {
  console.log(i);
  i++;
} while (i < 10);

In the above code, we defined the variable i , and then use the do...while loop to print out the value of i. Different from the while loop, in the first loop, the value of i is 0, which does not meet the loop condition, but because do...while# is used ##Loop, so the loop body will be executed at least once.

Summary

In JavaScript, loop operations are an essential part of the programming process. This article introduces commonly used loop methods, including for loop, forEach method, for...in loop , while loop and do...while loop, readers can choose different loop methods to implement their own code logic according to actual needs.

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