Substring with string javascript

String.prototype.substring()

The substring() method returns the part of the string from the start index up to and excluding the end index, or to the end of the string if no end index is supplied.

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Syntax

substring(indexStart) substring(indexStart, indexEnd) 

Parameters

The index of the first character to include in the returned substring.

The index of the first character to exclude from the returned substring.

Return value

A new string containing the specified part of the given string.

Description

substring() extracts characters from indexStart up to but not including indexEnd . In particular:

• If indexEnd is omitted, substring() extracts characters to the end of the string.
• If indexStart is equal to indexEnd , substring() returns an empty string.
• If indexStart is greater than indexEnd , then the effect of substring() is as if the two arguments were swapped; see example below.

Any argument value that is less than 0 or greater than str.length is treated as if it were 0 and str.length , respectively.

Any argument value that is NaN is treated as if it were 0 .

Examples

Using substring()

The following example uses substring() to display characters from the string ‘Mozilla’ :

const anyString = "Mozilla"; console.log(anyString.substring(0, 1)); // 'M' console.log(anyString.substring(1, 0)); // 'M' console.log(anyString.substring(0, 6)); // 'Mozill' console.log(anyString.substring(4)); // 'lla' console.log(anyString.substring(4, 7)); // 'lla' console.log(anyString.substring(7, 4)); // 'lla' console.log(anyString.substring(0, 7)); // 'Mozilla' console.log(anyString.substring(0, 10)); // 'Mozilla' 

Using substring() with length property

The following example uses the substring() method and length property to extract the last characters of a particular string. This method may be easier to remember, given that you don’t need to know the starting and ending indices as you would in the above examples.

const text = "Mozilla"; // Takes 4 last characters of string console.log(text.substring(text.length - 4)); // prints "illa" // Takes 5 last characters of string console.log(text.substring(text.length - 5)); // prints "zilla" 

The difference between substring() and substr()

There are subtle differences between the substring() and substr() methods, so you should be careful not to get them confused.

• The two parameters of substr() are start and length , while for substring() , they are start and end .
• substr() ‘s start index will wrap to the end of the string if it is negative, while substring() will clamp it to 0 .
• Negative lengths in substr() are treated as zero, while substring() will swap the two indexes if end is less than start .

Furthermore, substr() is considered a legacy feature in ECMAScript, so it is best to avoid using it if possible.

const text = "Mozilla"; console.log(text.substring(2, 5)); // "zil" console.log(text.substr(2, 3)); // "zil" 

Differences between substring() and slice()

The substring() and slice() methods are almost identical, but there are a couple of subtle differences between the two, especially in the way negative arguments are dealt with.

The substring() method swaps its two arguments if indexStart is greater than indexEnd , meaning that a string is still returned. The slice() method returns an empty string if this is the case.

const text = "Mozilla"; console.log(text.substring(5, 2)); // "zil" console.log(text.slice(5, 2)); // "" 

If either or both of the arguments are negative or NaN , the substring() method treats them as if they were 0 .

.log(text.substring(-5, 2)); // "Mo" console.log(text.substring(-5, -2)); // "" 

slice() also treats NaN arguments as 0 , but when it is given negative values it counts backwards from the end of the string to find the indexes.

.log(text.slice(-5, 2)); // "" console.log(text.slice(-5, -2)); // "zil" 

See the slice() page for more examples with negative numbers.

Replacing a substring within a string

The following example replaces a substring within a string. It will replace both individual characters and substrings. The function call at the end of the example changes the string Brave New World to Brave New Web .

// Replaces oldS with newS in the string fullS function replaceString(oldS, newS, fullS)  for (let i = 0; i  fullS.length; ++i)  if (fullS.substring(i, i + oldS.length) === oldS)  fullS = fullS.substring(0, i) + newS + fullS.substring(i + oldS.length, fullS.length); > > return fullS; > replaceString("World", "Web", "Brave New World"); 

Note that this can result in an infinite loop if oldS is itself a substring of newS — for example, if you attempted to replace ‘ World ‘ with ‘ OtherWorld ‘ here.

A better method for replacing strings is as follows:

function replaceString(oldS, newS, fullS)  return fullS.split(oldS).join(newS); > 

The code above serves as an example for substring operations. If you need to replace substrings, most of the time you will want to use String.prototype.replace() .

Specifications

Browser compatibility

BCD tables only load in the browser

See also

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String.prototype.substr()

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

The substr() method returns a portion of the string, starting at the specified index and extending for a given number of characters afterwards.

Note: substr() is not part of the main ECMAScript specification — it’s defined in Annex B: Additional ECMAScript Features for Web Browsers, which is normative optional for non-browser runtimes. Therefore, people are advised to use the standard String.prototype.substring() and String.prototype.slice() methods instead to make their code maximally cross-platform friendly. The String.prototype.substring() page has some comparisons between the three methods.

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Syntax

substr(start) substr(start, length) 

Parameters

The index of the first character to include in the returned substring.

The number of characters to extract.

Return value

A new string containing the specified part of the given string.

Description

A string’s substr() method extracts length characters from the string, counting from the start index.

• If start >= str.length , an empty string is returned.
• If start < 0 , the index starts counting from the end of the string. More formally, in this case the substring starts at max(start + str.length, 0) .
• If start is omitted or undefined , it’s treated as 0 .
• If length is omitted or undefined , or if start + length >= str.length , substr() extracts characters to the end of the string.
• If length < 0 , an empty string is returned.
• For both start and length , NaN is treated as 0 .

Although you are encouraged to avoid using substr() , there is no trivial way to migrate substr() to either slice() or substring() in legacy code without essentially writing a polyfill for substr() . For example, str.substr(a, l) , str.slice(a, a + l) , and str.substring(a, a + l) all have different results when str = «01234», a = 1, l = -2 — substr() returns an empty string, slice() returns «123» , while substring() returns «0» . The actual refactoring path depends on the knowledge of the range of a and l .

Examples

Using substr()

const aString = "Mozilla"; console.log(aString.substr(0, 1)); // 'M' console.log(aString.substr(1, 0)); // '' console.log(aString.substr(-1, 1)); // 'a' console.log(aString.substr(1, -1)); // '' console.log(aString.substr(-3)); // 'lla' console.log(aString.substr(1)); // 'ozilla' console.log(aString.substr(-20, 2)); // 'Mo' console.log(aString.substr(20, 2)); // '' 

Specifications

Browser compatibility

BCD tables only load in the browser

See also

Found a content problem with this page?

This page was last modified on Jun 20, 2023 by MDN contributors.

Your blueprint for a better internet.

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