What we can do with String.replace()

Here is the spec from Mozzila for String.replace().

The basic syntax for it is

var newString = str.replace(regexp/substr, newSubStr/function[, flags]);

It finds a match by either a regular expression or a string, and replaced it with the new string or the result of a function. For example,

var string = "hello world, user1 user";
var newString = string.replace("user", "thunder planet");
//the newString is "hello world, thunder planet1 user
var newString1 = string.replace(/user$/, "thunder planet")
//newString1 is "hello world, user1 thunder planet

The replacement string can include those special patterns

• $$ -- a '$'
• $& -- the matched substring
• $` -- the portion of string that precedes the matched string
• $' -- the portion of string that follows the matched string
• $n -- if the matched string is specified using a regular expression, it gives the group in the regular expression

Please see the following examples:

var string = "hello world user"
var newString = string.replace("user", "$$");
//newString is "hello world $"
var newString1 = string.replace("user", "$&");
//newString1 is the same with string
var newString2 = string.replace("user", "$`");
//newString2 is "hello world hello world"
var newString3 = string.replace("user", "$'");
//newString3 is "hello world "
string = "Hello, John Smith";
var newString4 = string.replace(/(\w+)\s(\w+)$/g, "$2,$1")
//newString4 is "Hello, Smith,John"

The definition to the replacement function depends on whether you are using regular expression and how many groups are in the regular expression. Basically, the replacement function is in form of

function replacer(str, p1, ..., pn, offset, s)

The first parameter is the matched string, and the following n (n can be zero) parameters represent the groups in the regular expression, offset is the offset for the matched string, and the last parameter is the whole string.

function replacer(str, p1, p2, offset, s){
    return "{" + str + "/" + p1 + "/" + p2 + "/" + offset + "/" + s + "}";
}

function replacer1(str, p1, offset, s){
    return "{" + str + "/" + p1 +  "/" + offset + "/" + s + "}";
}

"XXzzzz---Xz--XXzzz".replace(/(X+)(z+)/g, replacer)
//result is "{XXzzzz/XX/zzzz/0/XXzzzz---Xz--XXzzz}---{Xz/X/z/9/XXzzzz---Xz--XXzzz}--{XXzzz/XX/zzz/13/XXzzzz---Xz--XXzzz}"
"XXzzzz---Xz--XXzzz".replace(/(X+z+)/g, replacer)
//result is "{XXzzzz/XXzzzz/0/XXzzzz---Xz--XXzzz/undefined}---{Xz/Xz/9/XXzzzz---Xz--XXzzz/undefined}--{XXzzz/XXzzz/13/XXzzzz---Xz--XXzzz/undefined}
"XXzzzz---Xz--XXzzz".replace(/(X+z+)/g, replacer1)
//result is "{XXzzzz/XXzzzz/0/XXzzzz---Xz--XXzzz}---{Xz/Xz/9/XXzzzz---Xz--XXzzz}--{XXzzz/XXzzz/13/XXzzzz---Xz--XXzzz}"
"XXzzzz---Xz--XXzzz".replace("Xz", replacer1, "g")
//result is "X{Xz/1/XXzzzz---Xz--XXzzz/undefined}zzz---{Xz/9/XXzzzz---Xz--XXzzz/undefined}--X{Xz/14/XXzzzz---Xz--XXzzz/undefined}zz"
"XXzzzz---Xz--XXzzz".replace(/X+z+/g, replacer1)
//result is "{XXzzzz/0/XXzzzz---Xz--XXzzz/undefined}---{Xz/9/XXzzzz---Xz--XXzzz/undefined}--{XXzzz/13/XXzzzz---Xz--XXzzz/undefined}"

So the replacement function depends on the first argument in the String.replace function.