What is the difference between ng-if and ng-show/ng-hide

I'm trying to understand the difference between ng-if and ng-show/ng-hide, but they look the same to me.

Is there a difference that I should keep in mind choosing to use one or the other?

ngIf

The ngIf directive removes or recreates a portion of the DOM tree based on an expression. If the expression assigned to ngIf evaluates to a false value then the element is removed from the DOM, otherwise a clone of the element is reinserted into the DOM.

<!-- when $scope.myValue is truthy (element is restored) -->
<div ng-if="1"></div>

<!-- when $scope.myValue is falsy (element is removed) -->
<div ng-if="0"></div>

When an element is removed using ngIf its scope is destroyed and a new scope is created when the element is restored. The scope created within ngIf inherits from its parent scope using prototypal inheritance.

If ngModel is used within ngIf to bind to a JavaScript primitive defined in the parent scope, any modifications made to the variable within the child scope will not affect the value in the parent scope, e.g.

<input type="text" ng-model="data">
<div ng-if="true">
    <input type="text" ng-model="data">
</div>        

To get around this situation and update the model in the parent scope from inside the child scope, use an object:

<input type="text" ng-model="data.input">
<div ng-if="true">
    <input type="text" ng-model="data.input">
</div>

Or, $parent variable to reference the parent scope object:

<input type="text" ng-model="data">
<div ng-if="true">
    <input type="text" ng-model="$parent.data">
</div>

ngShow

The ngShow directive shows or hides the given HTML element based on the expression provided to the ngShow attribute. The element is shown or hidden by removing or adding the ng-hide CSS class onto the element. The .ng-hide CSS class is predefined in AngularJS and sets the display style to none (using an !important flag).

<!-- when $scope.myValue is truthy (element is visible) -->
<div ng-show="1"></div>

<!-- when $scope.myValue is falsy (element is hidden) -->
<div ng-show="0" class="ng-hide"></div>

When the ngShow expression evaluates to false then the ng-hide CSS class is added to the class attribute on the element causing it to become hidden. When true, the ng-hide CSS class is removed from the element causing the element not to appear hidden.

Maybe an interesting point to make, is the difference between priorities between both.

As far as I can tell, the ng-if directive has one of the highest (if not the highest) priority of all Angular directives. Which means: it will run FIRST before all other, lower prioritised, directives. The fact that it runs FIRST, means that effectively, the element is removed before any inner directives are processed. Or at least: that's what I make of it.

I observerd and used this in the UI I'm building for my current customer. The entire UI is quite heavily packed, and it had ng-show and ng-hide all over it. Not to go into too much detail, but I built a generic component, which could be managed using JSON config, so I had to do some switching inside the template. There is an ng-repeat present, and inside the ng-repeat, a table is shown, which has a lot of ng-shows, ng-hides and even ng-switches present. They wanted to show at least 50 repeats in the list, which would result in more or less 1500-2000 directives to be resolved. I checked the code, and the Java backend + custom JS on the front would take about 150ms to process the data, and then Angular would chew some 2-3 seconds on it, before displaying. The customer did not complain, but I was appalled :-)

In my search, I stumbled across the ng-if directive. Now, maybe it's best to point out that at the point of conceiving this UI, there was no ng-if available. Because the ng-show and ng-hide had functions in them, which returned booleans, I could easily replace them all with ng-if. By doing so, all inner directives seemed to be no longer evaluated. That meant that I dropped back to about a third of all directives being evaluated, and thus, the UI speeded up to about 500ms - 1 sec loading time. (I have no way to determine exact seconds)

Do note: the fact that the directives are not evaluated, is an educated guess about what is happening underneath.

So, in my opinion: if you need the element to be present on the page (ie: for checking the element, or whatever), but simply be hidden, use ng-show/ng-hide. In all other cases, use ng-if.