How to use the 'replace' feature for custom AngularJS directives? - angularjs

Question

Why does replace=true or replace=false not have any impact in the code below?
Why isn't the "some existing content" being displayed when replace=false?
Or putting it more humbly, can you kindly explain what is the replace=true/false feature in directives and how to use it?
Example
JS/Angular:
<script>
angular.module('scopes', [])
.controller('Ctrl', function($scope) {
$scope.title = "hello";
})
.directive('myDir', function() {
return {
restrict: 'E',
replace: true,
template: '<div>{{title}}</div>'
};
});
</script>
HTML:
<div ng-controller="Ctrl">
<my-dir><h3>some existing content</h3></my-dir>
</div>
See it in Plunker here:
http://plnkr.co/edit/4ywZGwfsKHLAoGL38vvW?p=preview

Answer 1

When you have replace: true you get the following piece of DOM:
<div ng-controller="Ctrl" class="ng-scope">
<div class="ng-binding">hello</div>
</div>
whereas, with replace: false you get this:
<div ng-controller="Ctrl" class="ng-scope">
<my-dir>
<div class="ng-binding">hello</div>
</my-dir>
</div>
So the replace property in directives refer to whether the element to which the directive is being applied (<my-dir> in that case) should remain (replace: false) and the directive's template should be appended as its child,
OR
the element to which the directive is being applied should be replaced (replace: true) by the directive's template.
In both cases the element's (to which the directive is being applied) children will be lost. If you wanted to perserve the element's original content/children you would have to translude it. The following directive would do it:
.directive('myDir', function() {
return {
restrict: 'E',
replace: false,
transclude: true,
template: '<div>{{title}}<div ng-transclude></div></div>'
};
});
In that case if in the directive's template you have an element (or elements) with attribute ng-transclude, its content will be replaced by the element's (to which the directive is being applied) original content.
See example of translusion http://plnkr.co/edit/2DJQydBjgwj9vExLn3Ik?p=preview
See this to read more about translusion.

Answer 2

replace:true is Deprecated
From the Docs:
replace ([DEPRECATED!], will be removed in next major release - i.e. v2.0)
specify what the template should replace. Defaults to false.
true - the template will replace the directive's element.
false - the template will replace the contents of the directive's element.
-- AngularJS Comprehensive Directive API
From GitHub:
Caitp-- It's deprecated because there are known, very silly problems with replace: true, a number of which can't really be fixed in a reasonable fashion. If you're careful and avoid these problems, then more power to you, but for the benefit of new users, it's easier to just tell them "this will give you a headache, don't do it".
-- AngularJS Issue #7636
Update
Note: replace: true is deprecated and not recommended to use, mainly due to the issues listed here. It has been completely removed in the new Angular.
Issues with replace: true
Attribute values are not merged
Directives are not deduplicated before compilation
transclude: element in the replace template root can have unexpected effects
For more information, see
AngularJS $compile Service API Reference - Issues with replace:true