Final View:

Steps to do it?

Step 1: Create a JavaScript code file to override default view
Step 2: Upload JavaScript file to Master Page Gallery –> Display Templates folder
Step 3: Set JS Link Property of List view web part to Url of uploaded JavaScript file.

Step 1: Create a JavaScript code file to override default view
In the first step, we need to develop the JavaScript code file that will contain the customization logic.

Simple example:
The following code shows a simple function that will display description field in the list in italics.

(function () {
var overrideCtx = {};
overrideCtx.Templates = {};
overrideCtx.Templates.Fields = {‘Description’: { ‘View’: ‘<i><#=ctx.currentItem.Description#></i>’ }
};
SPClientTemplates.TemplateManager.RegisterTemplateOverrides(overrideCtx);
})();

overrideCtx variable holds the current context of the list item.
Templates.Fields property provides access to individual list fields.
RegisterTemplateOverrides function call registers the templates for your list to use.

Main Code to customize PercentComplete column in task list
CustomizeTaskCompletionView.js
(function () {

// Initialize variable that will store overrides objects.
var overrideCtx = {};

overrideCtx.Templates = {};
overrideCtx.Templates.Header = “”;
overrideCtx.Templates.Footer = “”;
overrideCtx.Templates.Fields = { ‘PercentComplete’: { ‘View’: CustomProgressBar } };

// Register template overrides.
SPClientTemplates.TemplateManager.RegisterTemplateOverrides(overrideCtx); })();
function CustomProgressBar(ctx) {
var percentage = ctx.CurrentItem.PercentComplete;
var percentageNr = ctx.CurrentItem.PercentComplete.replace(” %”, “”);
var duedate = new Date(ctx.CurrentItem.DueDate);
var color = getCompletion(percentageNr, duedate);
return “<div style=\”background: #F3F3F3; display:block; height: 15px; width: 120px;\”><div    style=\”background: ” + color + “; height: 100%; width: ” + percentageNr + “%;\”></div></div> ” + percentage + “”;

}

function getCompletion(progress, duedate) {
var datenow = new Date();
if (parseInt(progress) < 100 && duedate.getTime() < datenow.getTime())
return “Red”;
else
return “Green”;
}

Step 2: Upload JavaScript file to Master Page Gallery à Display Templates folder
Navigate to Site Settings –> Master Pages and page layouts –> Display templates folder

Upload JavaScript file CustomizeTaskCompletionView.js created in step 1 to this folder.

While uploading JavaScript file choose:

Content type –> JavaScript Display Template
Target Control Type –> View
Standalone –> Override
Target Scope –> /

Step 3: Set JS Link Property of List view web part to Url of uploaded JavaScript file.
Navigate to Task list and edit list view WebPart, I have opened Workflow Tasks list

You will see JSLink property under Miscellaneous section

If JSLink property is missing, In Office 365, go to your SharePoint Admin Center –> Settings and enable custom script.

It may take upto 24 hours before you start seeing JSLink property once you enable it.
Once property is visible, provide uploaded JavaScript URL. We cannot give absolute path, we will have to use site collection token representation like this:

~sitecollection/_catalogs/masterpage/Display Templates/CustomizeTaskCompletionView.js

To learn more about URLs and tokens in SharePoint 2013. Refer this MSDN link: https://msdn.microsoft.com/library/office/ms431831.aspx

Save changes and add new task to this list. It will look like this: