Why does `String.prototype.concat` not modify the string in place?

Question

I've been trying to create a news box for my website today and I have a problem. Let me explain what happens. I create a 2d array that contains the news (date and the news). I then loop through it to construct the news_string and then use that for the DIV's innerHTML. I have put a very simple version of it below

for (var i = 0; i < news.length; i++) 
{
news_string.concat(news[i][1],"<br>");
}

document.getElementById("news-content").innerHTML = news_string;

However nothing appears. I have cut it down to the very minimal. No result. I have used alerts. Nothing appears. The news_string is blank regardless of the fact I put data into it. And even if I do gain a string nothing appears in the DIV box. What's causing this massive break?

However if I do this. news_string = news_string + news[i][1] + "<br>"; alert(news_string) The string completely appears — Jeremy Beare
– Jeremy Beare, Commented Feb 16, 2014 at 11:14
@JeremyBeare: Strings are immutable in JavaScript. news_string.concat returns a new string, so the code in your loop isn't really changing anything. — Blender
– Blender, Commented Feb 16, 2014 at 11:15
What about using news_string = news_string.concat(news[i][1],"<br>"); ? — Sergio
– Sergio, Commented Feb 16, 2014 at 11:16

Sergio · Accepted Answer · 2014-02-16 11:16:50Z

1

The concat method returns a value, you have no variable assignement there to catch it...

From the docs (notice the bold part):

The concat() method combines the text of two or more strings and returns a new string.

So you should use:

news_string = news_string.concat(news[i][1],"<br>");

answered Feb 16, 2014 at 11:16

Sergio

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18 Comments

Niet the Dark Absol Over a year ago

Oh, pfft, how did I not notice that?

Blender Over a year ago

Or just news.map(function(thing) { return thing[1]; }).join('<br>').

Jeremy Beare Over a year ago

Makes perfect sense. Can't believe I made that error. So the string now has values. However the problem still persists that the innerHTML is blank. This is the code I pass into the innerHTML document.getElementById("news-content").innerHTML = news_string;

Sergio Over a year ago

@JeremyBeare, please post a sample of the array.

Sergio Over a year ago

@JeremyBeare, in the inline javascript remove the (). refer just to the function loadNews in the parameter without the (). I missleaded you in my fast example before sorry. Check here: jsfiddle.net/C7nh3

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