In JavaScript, there are four common ways to add elements to an array: 1. Push() adds at the end and returns a new length; 2. Unshift() adds at the beginning and returns a new length; 3. Direct assignment through the index is suitable for specified positions, but may generate empty spaces; 4. splice() can insert elements at any position and modify the original array. These methods will change the original array, and you need to choose the appropriate method according to the specific needs when using it.

In JavaScript, adding elements to an array is a very common operation. Although there are many methods, different scenarios are suitable for different methods. The key is to be clear about the characteristics and application of each method.

1. Use push() method to add elements to the end of the array

This is one of the most commonly used methods, push() can append one or more elements at the end of the array and return a new length.

 let arr = [1, 2, 3];
arr.push(4);
console.log(arr); // [1, 2, 3, 4]

• Supports adding multiple elements at once:

  

   arr.push(4, 5, 6);

• This method will change the original array , so be careful of side effects when using it.

2. Use unshift() to add elements at the beginning

If you need to insert elements at the beginning of the array, you can use unshift() :

 let arr = [2, 3, 4];
arr.unshift(1);
console.log(arr); // [1, 2, 3, 4]

• Adding multiple elements is also supported:

   arr.unshift(0, 1);

• Like push() , this method also modifies the original array .

3. Direct assignment through index (applicable to specified locations)

If you know where to insert, you can assign values ??directly through the index:

 let arr = [1, 2, 3];
arr[3] = 4;
console.log(arr); // [1, 2, 3, 4]

• If the index exceeds the current array length, a "empty" will appear in the middle of the array (i.e. empty ):

   arr[5] = 6; // arr is now [1, 2, 3, 4, empty, 6]

• This method will not automatically fill in the gaps in the middle, so you need to pay attention to it when using it.

4. Use splice() to insert elements anywhere

splice() is a powerful method that can be used to delete, replace or insert elements.

 let arr = [1, 2, 4, 5];
// Insert '3' in index 2
arr.splice(2, 0, 3);
console.log(arr); // [1, 2, 3, 4, 5]

Parameter description:

• The first parameter: Starting index

• The second parameter: the number of elements to be deleted (0 means not to delete)

• Third and later: Elements to be added

• This method will also modify the original array and return the deleted array of elements (if not deleted, it will return an empty array).

Basically these commonly used ways to add elements. Different methods have different uses and characteristics, such as whether to modify the original array, insertion position, etc., and choose the most appropriate method according to the specific needs.

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