JavaScript replace \n with <br />

You need the /g for global matching

replace(/\n/g, "<br />");

This works for me for \n - see this answer if you might have \r\n

NOTE: The dupe is the most complete answer for any combination of \r\n, \r or \n

var messagetoSend = document.getElementById('x').value.replace(/\n/g, "<br />");
console.log(messagetoSend);

<textarea id="x" rows="9">
    Line 1
    
    
    Line 2
    
    
    
    
    Line 3
</textarea>

UPDATE

It seems some visitors of this question have text with the breaklines escaped as

some text\r\nover more than one line"

In that case you need to escape the slashes:

replace(/\\r\\n/g, "<br />");

NOTE: All browsers will ignore \r in a string when rendering.

Handles either type of line break

str.replace(new RegExp('\r?\n','g'), '<br />');

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