javascript

String - Manipulation Methods
Chapter 5: Reference Types
Several methods manipulate the values of strings. The first of these methods is concat() , which is used
to concatenate one or more strings to another, returning the concatenated string as the result. Consider
the following example:
var stringValue = “hello “;
var result = stringValue.concat(“world”);
alert(result);
//”hello world”
alert(stringValue); //”hello”
The result of calling the concat() method on stringValue in this example is “ hello world “ — the
value of stringValue remains unchanged. The concat() method accepts any number of arguments, so
it can create a string from any number of other strings, as shown here:
var stringValue = “hello “;
var result = stringValue.concat(“world”, “!”);
alert(result);
alert(stringValue);
//”hello world!”
//”hello”
This modified example concatenates “ world ” and “ ! ” to the end of “ hello “ . Although the concat()
method is provided for string concatenation, the addition operator ( + ) is used more often and, in most
cases, actually performs better than the concat() method even when concatenating multiple strings.
ECMAScript provides three methods for creating string values from a substring: slice() , substr() ,
and substring() . All three methods return a substring of the string they act on, and all accept either
one or two arguments. The first argument is the position where capture of the substring begins; the
second argument, if used, indicates where the operation should stop. For slice() and substring() ,
this second argument is the position before which capture is stopped (all characters up to this point are
included except the character at that point). For substr() , the second argument is the number of
characters to return. If the second argument is omitted in any case, it is assumed that the ending position
is the length of the string. Just as with the concat() method, slice() , substr() , and substring() do
not alter the value of the string itself — they simply return a primitive string value as the result, leaving
the original unchanged. Consider this example:
var stringValue = “hello world”;
alert(stringValue.slice(3)); //”lo world”
alert(stringValue.substring(3)); //”lo world”
alert(stringValue.substr(3)); //”lo world”
alert(stringValue.slice(3, 7)); //”lo w”
alert(stringValue.substring(3,7)); //”lo w”
alert(stringValue.substr(3, 7)); //”lo worl”
In this example, slice() , substr() , and substring() are used in the same manner, and in most cases
return the same value. When given just one argument, 3 , all three methods return “ lo world ” , because
the second “ l ” in “ hello ” is in position 3. When given two arguments, 3 and 7 , slice() and
substring() return “ lo w ” (the “ o ” in “ world ” is in position 7, so it is not included), while substr()
returns “ lo worl ” because the second argument specifies the number of characters to return.
There are different behaviors for these methods when an argument is a negative number. For the
slice() method, a negative argument is treated as the length of the string plus the negative argument.
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