In the current version of JavaScript, you can do this by escaping the newlines at the ends of lines, which is a bit better but note that leading whitespace will be included in the string, and you still have to use concatenation to swap in your values:
var rightPane = $("#right");
// Draw the right pane
rightPane.html('<h2>' + task.task + '<h2> <hr />\
data1: <input type="text" id="data1" value="' + task.data1 + '" /> <br />\
data2: <input type="text" id="data2" value="' + task.data2 + '" /> <br />\
data1: <input type="text" id="data3" value="' + task.data3 + '" /> <br />\
<input type="button" id="subUpdated" value="Save">');
In the next version of JavaScript, "ES6", we'll have template strings which can be multi-line and which let you easily swap in text using ${...}
:
// REQUIRES ES6
var rightPane = $("#right");
// Draw the right pane
rightPane.html(`
<h2>${task.task}<h2> <hr />
data1: <input type="text" id="data1" value="${task.data1}" /> <br />
data2: <input type="text" id="data2" value="${task.data2}" /> <br />
data1: <input type="text" id="data3" value="${task.data3}" /> <br />
<input type="button" id="subUpdated" value="Save">
`);
(Again leading whitespace is included in the string.)
To do that before ES6, you can use any of several templating libraries (Handlebars, Mustache, RivetsJS).
For a really simple version, you could use the function I wrote for another question.