string.substring();

substring Method
Returns the substring at the specified location within a String object.

strVariable.substring(start, end)
"String Literal".substring(start, end)

Arguments
start

The zero-based index integer indicating the beginning of the substring.

end

The zero-based index integer indicating the end of the substring.

Remarks
The substring method returns a string containing the substring from start up to, but not including, end.

The substring method uses the lower value of start and end as the beginning point of the substring. For example, strvar.substring(0, 3) and strvar.substring(3, 0) return the same substring.

If either start or end is NaN or negative, it is replaced with zero.

The length of the substring is equal to the absolute value of the difference between start and end. For example, the length of the substring returned in strvar.substring(0, 3) and strvar.substring(3, 0) is three.

Example
The following example illustrates the use of the substring method.

function SubstringDemo(){
   var ss;                         //Declare variables.
   var s = "The rain in Spain falls mainly in the plain..";
   ss = s.substring(12, 17);   //Get substring.
   return(ss);                     //Return substring.
}

原文转自：http://www.ltesting.net