JavaScript String Methods

String methods help you to work with strings.

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Finding a String in a String

The indexOf() method returns the index of (the position of) the first occurrence of a specified text in a string:

Example

var str = "Please locate where 'locate' occurs!";
var pos = str.indexOf("locate"); 

 

The lastIndexOf() method returns the index of the last occurrence of a specified text in a string:

Example

var str = "Please locate where 'locate' occurs!";
var pos = str.lastIndexOf("locate"); 

 

Both the indexOf(), and the lastIndexOf() methods return -1 if the text is not found.

	JavaScript counts positions from zero. 0 is the first position in a string, 1 is the second, 2 is the third ...

Both methods accept a second parameter as the starting position for the search.

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Searching for a String in a String

The search() method searches a string for a specified value and returns the position of the match:

Example

var str = "Please locate where 'locate' occurs!";
var pos = str.search("locate"); 

 

The two methods, indexOf() and search(), are equal.
They accept the same arguments (parameters), and they return the same value.
The two methods are equal, but the search() method can take much more powerful search values.
You will learn more about powerful search values in the chapter about regular expressions.

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Extracting String Parts

There are 3 methods for extracting a part of a string:

• slice(start, end)
• substring(start, end)
• substr(start, length)

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The slice() Method

slice() extracts a part of a string and returns the extracted part in a new string.
The method takes 2 parameters: the starting index (position), and the ending index (position).
This example slices out a portion of a string from position 7 to position 13:

Example

var str = "Apple, Banana, Kiwi";
var res = str.slice(7,13);

The result of res will be:

Banana

 

If a parameter is negative, the position is counted from the end of the string.
This example slices out a portion of a string from position -12 to position -6:

Example

var str = "Apple, Banana, Kiwi";
var res = str.slice(-12,-6);

The result of res will be:

Banana

 

If you omit the second parameter, the method will slice out the rest of the string:

Example

var res = str.slice(7); 

 

or, counting from the end:

Example

var res = str.slice(-12); 

 

The substring() Method

substring() is similar to slice().
The difference is that substring() cannot accept negative indexes.

Example

var str = "Apple, Banana, Kiwi";
var res = str.substring(7,13);

The result of res will be:

Banana

 

If you omit the second parameter, substring() will slice out the rest of the string.

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The substr() Method

substr() is similar to slice().
The difference is that the second parameter specifies the length of the extracted part.

Example

var str = "Apple, Banana, Kiwi";
var res = str.substr(7,6);

The result of res will be:

Banana

 

If the first parameter is negative, the position counts from the end of the string.
The second parameter can not be negative, because it defines the length.
If you omit the second parameter, substr() will slice out the rest of the string.

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The replace() method can also take a regular expression as the search value.

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Converting to Upper and Lower Case

A string is converted to upper case with toUpperCase():

Example

var text1 = "Hello World!";       // String var text2 = text1.toUpperCase();  // text2 is text1 converted to upper

 

A string is converted to lower case with toLowerCase():

Example

var text1 = "Hello World!";       // String var text2 = text1.toLowerCase();  // text2 is text1 converted to lower 

 

The concat() Method

concat() joins two or more strings:

Example

var text1 = "Hello";
var text2 = "World";
text3 = text1.concat(" ",text2); 

 

The concat() method can be used instead of the plus operator. These two lines do the same:

Example

var text = "Hello" + " " + "World!";
var text = "Hello".concat(" ","World!");

	All string methods return a new string. They don't modify the original string. Formally said: Strings are immutable: Strings cannot be changed, only replaced.

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Extracting String Characters

There are 2 safe methods for extracting string characters:

• charAt(position)
• charCodeAt(position)

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The charAt() Method

The charAt() method returns the character at a specified index (position) in a string:

Example

var str = "HELLO WORLD";
str.charAt(0);            // returns H 

 

The charCodeAt() Method

The charCodeAt() method returns the unicode of the character at a specified index in a string:

Example

var str = "HELLO WORLD";

str.charCodeAt(0);         // returns 72

  

Accessing a String as an Array is Unsafe

You might have seen code like this, accessing a string as an array:

var str = "HELLO WORLD";

str[0];                   // returns H

This is unsafe and unpredictable:

• It does not work in all browsers (not in IE5, IE6, IE7)
• It makes strings look like arrays (but they are not)
• str[0] = "H" does not give an error (but does not work)

If you want to read a string as an array, convert it to an array first.

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Converting a String to an Array

A string can be converted to an array with the split() method:

Example

var txt = "a,b,c,d,e";   // String txt.split(",");          // Split on commas txt.split(" ");          // Split on spaces txt.split("|");          // Split on pipe 

 

If the separator is omitted, the returned array will contain the whole string in index [0].
If the separator is "", the returned array will be an array of single characters:

Example

var txt = "Hello";       // String txt.split("");           // Split in characters