Concatenating strings with `if` statements in JavaScript
I'm attempting to set up a script to concatenate some variables inside a string if they exist, in order to place the appropriate metadata tags into a rendered HTML document.
My concatenation code is:
data = "<html>\n<head>\n" + "</head>\n<body>\n\n" + paras.join("\n\n") + "\n\n</body>\n</html>";
I'm trying to add if statements like the following into it (between the first and second items):
if (typeof metadata_title !== "undefined") {
"<title>" + metadata_title + "</title>\n"
}
if (typeof metadata_author !== "undefined") {
"<meta name=\"author\" content=\"" + metadata_author + "\"></meta>\n"
}
if (typeof metadata_date !== "undefined") {
"<meta name=\"date\" content=\"" + metadata_date + "\"></meta>\n"
}
But I can't add any of these statements directly into the concatenation code
(it throws an error: Unexpected token ().开发者_开发百科
How best would I go about adding statements such as these into my concatenation string?
I'd use a ternary operator:
data = "<html>\n"
+ "<head>\n"
+ ( typeof metadata_title !== "undefined" ? "<title>" + metadata_title + "</title>\n" : "" )
+ ( typeof metadata_author !== "undefined" ? "<meta name=\"author\" content=\"" + metadata_author + "\"></meta>\n" : "" )
+ ( typeof metadata_date !== "undefined" ? "<meta name=\"date\" content=\"" + metadata_date + "\"></meta>\n" : "" )
+ "</head>\n"
+ "<body>\n"
+ "\n"
+ paras.join("\n\n")
+ "\n"
+ "\n"
+ "</body>\n"
+ "</html>"
;
data = "<html>\n<head>\n"
+ (
typeof metadata_title !== "undefined" ?
"<title>" + metadata_title + "</title>\n" :
""
)
+ (
typeof metadata_author !== "undefined" ?
"<meta name=\"author\" content=\"" + metadata_author + "\"></meta>\n" :
""
)
+ (
typeof metadata_date !== "undefined" ?
"<meta name=\"date\" content=\"" + metadata_date + "\"></meta>\n" :
""
)
+ "</head>\n<body>\n\n"
+ paras.join("\n\n")
+ "\n\n</body>\n</html>";
I might do something a little different (a little more akin to templating), mainly because I hate concatenated HTML done with Javascript:
var metadata_title = "Hello";
var metadata_author = "Me";
var metadata_date = "2011-09-07";
var template = "<html>\
<head>\
<title>#title#</title>\
<meta name=\"author\" content=\"#author#\"></meta>\
<meta name=\"date\" content=\"#date#\"></meta>\
</head>\
<body>\
</body>\
</html>";
var data = template.replace("#title#", metadata_title != undefined ? metadata_title : "")
.replace("#author#", metadata_author != undefined ? metadata_author : "")
.replace("#date#", metadata_date != undefined ? metadata_date : "");
Sure, there's a very small amount of additional overhead, but to me, it's way more readable.
Build up the entire document into an array, then join with a "\n" at the end. (The rationale for this is of course to not have lots of new lines scattered all about! And if you are on IE7 or less, Array#join is considerably faster than repeated string concatenation.)
Code here: http://jsfiddle.net/ZCbCZ/
UPDATE I forgot to include the "paras" in the first fiddle. The code with the paras is here: http://jsfiddle.net/U8325/1/
For those not wishing to click through and try it out, here is the script:
// Not going to define metadata_author just to be saved by typeof :-)
var metadata_title = "Hello";
var metadata_date = "2011-09-07";
// Okay 3 paras for fun
var paras = ["<p>paragraph1</p>", "<p>paragraph2</p>", "<p>paragraph3</p>"];
data = ["<html>", "<head>"]
if (typeof metadata_title !== "undefined") {
data.push("<title>" + metadata_title + "</title>");
}
if (typeof metadata_author !== "undefined") {
data.push("<meta name=\"author\" content=\"" + metadata_author + "\"></meta>");
}
if (typeof metadata_date !== "undefined") {
data.push("<meta name=\"date\" content=\"" + metadata_date + "\"></meta>");
}
data.push("</head>");
data.push("<body>");
paras.forEach(function (p) {data.push(p);}); // Requires ES5; use a for-loop if you don't have it
data.push("</body>");
data.push("<html>");
data = data.join("\n");
alert(data);
I liked the readability of Demian Brecht answer, but I would only change the string for a regex instead, because the replace() function only replaces the first match (see more here: JavaScript .replace only replaces first Match)
var metadata_title = "Hello";
var metadata_author = "Me";
var metadata_date = "2011-09-07";
var template = "<html>\
<head>\
<title>#title#</title>\
<meta name=\"author\" content=\"#author#\"></meta>\
<meta name=\"date\" content=\"#date#\"></meta>\
</head>\
<body>\
</body>\
</html>";
var data = template.replace(/#title#/g, metadata_title != undefined ? metadata_title : "")
.replace(/#author#/g, metadata_author != undefined ? metadata_author : "")
.replace(/#date#/g, metadata_date != undefined ? metadata_date : "");
加载中,请稍侯......
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