Difference between string slicing and substring methods

JavaScript is a dynamic and most popular programming language that can be used on both the client and server sides. JavaScript is used to create interactive web pages. It has many frameworks, such as React JS, Angular JS, Node JS, etc.

JavaScript contains many built-in functions to perform various tasks. There are functions for manipulating strings. Str.slice and str.substring are two inbuit functions that can operate on strings. Although the functionality of these two functions is almost similar, there are some differences between them

String.slice() method

This method returns a portion of the string based on the specified index. The syntax of the slice method is as follows -

string.slice (start, end)

This method has two parameters, namely start and end. Start specifies the index from which it should start, and end index tells the last index of the string to be returned. The ending index is not included when returning the string. Ending is not necessary. If end is not specified, returns the entire string starting at the specified starting index. Here, the index starts from "0".

String.substring() method

This method is also similar to the slicing method, returning a part of the string as a new string. The syntax of this method is as follows -

string.substring (start, end)

Here, start and end are two parameters, indicating the index of the string that should be returned. If the end index is not specified, the entire string starting at the specified start index is returned. The ending index is not included when returning the string. The index starts from "0".

Consider the string "Good Morning" with a string length of 12. Let us understand the functionality of these methods with the following examples -

const greeting = 'Good Morning'

Case 1

If the starting index is positive, both methods give the same output.

greeting.slice (5)  		//  'Morning'
greeting.substring (5)  	// 'Morning'

Here, the index is set to 5, so the string starting from the 5th position of the main string is returned.

Case 2

If the given index is negative, these two methods will give different output, as shown below.

greeting.slice (-7)  // 'Morning'
greeting.substring (-7)  // 'Good Morning'

In the slice method, when a negative index is given, it counts from the end of the string and returns the string starting at that index position. Within a substring, any negative number is treated as zero. So it starts at index 0 and returns the entire string.

Case 3

Both methods return an empty string when passed a string length.

greeting.slice (12)  // ''
greeting.substring (12)  // ''

Case 4

Start index is greater than end index

greeting.slice (9, 5)  // ''
greeting.substring (9, 5)  // 'Morn'

The Slice method returns an empty string, while the substring method swaps these indices and returns a string. Excludes ending index.

Case 5

When the start and end indexes are both negative

greeting.slice (-12, -8)  // 'Good'
greeting.substring (-12, -8)  // ''

The Slice method counts from the end of the string to the specified index and returns it. Since both indices are negative and the substring method treats them as zero, it starts at zero and ends at zero. Therefore, an empty string is returned.

Case 6

When the starting index is negative and the ending index is positive

greeting.slice (-3, 5)  // ''
greeting.substring (-3, 5)  // 'Good'

Here, since the end index appears before the start index, the slice method returns an empty string. But the substring method treats negative indexes as zero and starts from the 0 index to the 5th index excluding it. Therefore, it returns "Good".

Case 7

When the start index is positive but the end index is negative

greeting.slice (5, -3)  // 'Morn'
greeting.substring (5, -3)  // 'Good'

The Slice method returns "Morn" as output and the Substring method returns "Good". The Substring method treats the end index as zero, and since the start index (5) is greater than the end index (0), it simply swaps their positions (0,5) and returns the string.

Slicing and substring methods

The following table highlights the important differences between slicing methods and substring methods -

Slicing method	Substring method
It returns a part of the string	Also used to extract part of a string
It allows negative indexing and returns the string count from behind	If a negative index is given, it is treated as the 0 (zero) index position.
If the starting index is greater than the ending index, an empty string is returned	If the starting index is greater than the ending index, it just swaps their positions and returns a string based on that result

in conclusion

String.slice and string.substring methods are used for string operations. Depending on the parameters and certain situations, they are all similar. The first major difference between them is that their indexes are negative, as shown in the example above. The second one is when the start index is greater than the end index.

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