When you write JavaScript, you need to know what string manipulation methods/functions are available.

JavaScript substr() Method

The substr() method extracts a specified number of characters in a string, from a start index. The syntax of substr() is given below:

stringObjectToTakeAPartOf.substr(start-index,length)

• start-index: (Required - Numeric Value) - where to start the extraction
• length: (Optional - Numeric Value) - how many characters to extract
  

Notes about substr:

• To extract characters from the end of the string, use a negative start-index. (Internet Explorer returns the whole string which is wrong.)
  
• If the length parameter is omitted, substr method extracts to the end of the string.
• substr() is not supported by Netscape 2, Netscape 3, Internet Explorer 3 and Opera 3.

substr Example:

<script type="text/javascript">

var str = "Hello World";
document.write(str.substr(4,4));

</script>

The output of the above code is:

o wo

Since substr() is not cross-browser, I never use it. Instead, I use the javascript's substring() function. 

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