Difference between substring and slice JavaScript String

Today’s video will cover the difference between substring and slice methods available on JavaScript string.


let fruits = 'Banana,Strawberry,Apple,Peach';
//Length of fruits
let length = fruits.length;
console.log('Length of fruits: ', length);
//Without endIndex (default endIndex = length - 1)
console.log('Without endIndex => ', fruits.substring(2));
//startIndex and endIndex are same (returns empty string)
console.log('Same startIndex & endIndex => ',fruits.substring(3,3));
//index > length of the string (returns the length of the string)
console.log('startIndex > length => ',fruits.substring(length+1));
console.log('endIndex > length => ',fruits.substring(1,length+2));
//startIndex > endIndex (indexes are swapped)
console.log('startIndex > endIndex => ',fruits.substring(10, 2));
//startIndex || endIndex === NaN (index is treated as 0)
//startIndex || endIndex < 0 (index is treated as 0)
console.log('Invalid indexes => ',fruits.substring(-1));
console.log('Invalid indexes => ',fruits.substring(NaN));

Full Transcript

00:00 Hi there. Welcome to the JavaScript series and I'm Deeksha Sharma from Bonsaiilabs. Today. In this video, we are going to cover the difference between the substring() and slice() methods available on JavaScript string. I've already covered a separate video on substring() but in short given this fruits variable containing string that is comma separated fruits names. If we want to know if there is a way to extract just the Apple,Peach substring() from the given fruits string, we can use substring() method that has two arguments, a start index to indicate the index to start the search from and end index until which the string would be extracted. The end index is never included in the extracted string and it's optional. When not provided it extracts till the end of the string. This is a quick usage of substring() with just the start index 18 and it returns Apple,Peach.

00:48 Now there is another method to achieve the exact same result and that is the slice() method. It has the same syntax and accept the same two arguments, a start index and end index, and when we call slice() method on fruits string, it returns the exact same result. Now, what's the difference between the two? Let's look at few of them. With substring() method. If a start index is greater than the end index, which one would only do it by mistake, it swaps the start index with the end index. For example, Hello.substring(10,2) is the same as Hello.substring(2,10). It extracts the characters starting from index 2 till the end of the string and returns llo but with slice() method, if start index is greater than the end index, it returns an empty string. Let's look at this example.

01:34 If we call Hello.slice(10,2), it return an empty string. Next while using the substring() method, if the start or end index is not a number or a negative number, then that particular index is treated as zero. For example, calling Hello.substring(NaN) returns the entire string. This is because the start index is treated as zero and there is no end index, so all the characters till the end of the string are extracted and returned. Similar is the case with negative indexes. Any negative number that is passed as a start or end index will be treated as zero. Now with slice() method. The rule for passing, not a number is same as substring() method, but if the start or end index are negative, their corresponding values will be calculated as length of string plus start index and length of string plus end index depending upon which index was negative.

02:28 So in this example, using slice() with NaN returns the string Hello, because NaN is again treated as zero and with no end index it extracted till the end of the string. Using slice() with negative start index will calculate the value of start index. It does that by adding the length of the string, which is five and minus two which is the start index here. The addition of both is three, so it extracts the characters from three to four, which is L. slice() with negative end index will calculate the value of index as five minus three which is two. Now two becomes the end index and it extracts the characters from zero to two which is HE. That's all I wanted to cover in this short video. I hope it was quick and useful for you. Thanks for watching this video. If you would like to take your JavaScript knowledge to the next level and want to know and understand how browsers execute JavaScript and the role of event loop, check out our new course, which is live. I've added the link in the description below. Friends, keep learning and share your knowledge. I'll see you in the next video.