How do you use if and else bindings in Knockout to show or hide elements?

1 What are if and else bindings?

If and else bindings are special types of control flow bindings in Knockout. They let you conditionally render or remove elements from the DOM based on a boolean expression. For example, you can use an if binding to show a message only if the user is logged in, or an else binding to show a different message if the user is not logged in. The syntax of these bindings is similar to the JavaScript if and else statements, but with data-bind attributes. For example:

<div data-bind="if: isLoggedIn">

<p>Welcome, <span data-bind="text: username"></span>!</p>

</div>

<div data-bind="else">

<p>Please log in to access this page.</p>

</div>

2 How do if and else bindings work?

When you use an if or an else binding, Knockout evaluates the expression inside the binding and decides whether to render or remove the element and its children. If the expression is true, Knockout renders the element and applies bindings to its children. If the expression is false, Knockout removes the element and its children from the DOM. This means that the element and its children are not part of the page at all, and do not take up any space or resources. Knockout also updates the element and its children whenever the expression changes, so you can create dynamic UIs that respond to user actions or data changes.

3 How are if and else bindings different from visible and ifnot bindings?

You might wonder why you need if and else bindings when you already have visible and ifnot bindings in Knockout. Visible and ifnot bindings are also control flow bindings, but they work differently from if and else bindings. Visible and ifnot bindings do not remove elements from the DOM, but only change their CSS display property to none or block. This means that the elements and their children are still part of the page, but are hidden from view. Visible and ifnot bindings also apply bindings to their children regardless of their visibility, which can affect performance and functionality. For example, if you have a hidden element with a click binding, it can still be triggered by keyboard events or other scripts.

4 When should you use if and else bindings?

You should use if and else bindings when you want to completely remove or add elements to the DOM based on some condition. This can improve performance, accessibility, and user experience, as you avoid rendering unnecessary elements or loading resources that are not needed. You should also use if and else bindings when you want to ensure that the bindings of the elements and their children are only applied when they are visible, as this can prevent unwanted side effects or errors. For example, you can use an if binding to load a chart only when the user clicks a button, or an else binding to show a loading indicator while an AJAX request is pending.

5 How do you use nested if and else bindings?

You can use nested if and else bindings to create more complex conditional logic in your UI. For example, you can use an if binding inside an else binding to show different messages based on different criteria. However, you should be careful not to create too many nested levels, as this can make your code harder to read and maintain. You should also avoid using conflicting or redundant conditions, as this can cause unexpected results or errors. For example, you should not use an if binding and an else binding with the same condition, as this will always render one of them and remove the other. Instead, you should use an ifnot binding or a negated condition. For example:

<div data-bind="if: hasItems">

<p>You have <span data-bind="text: itemCount"></span> items in your cart.</p>

</div>

<div data-bind="else">

<p>Your cart is empty.</p>

<!-- Don't do this: -->

<!-- <div data-bind="if: hasItems"> -->

<!-- <p>Some other message.</p> -->

<!-- </div> -->

<!-- Do this instead: -->

<div data-bind="if: hasDiscount">

<p>You have a <span data-bind="text: discountAmount"></span> discount.</p>

</div>

</div>

6 How do you use multiple conditions with if and else bindings?

You can use multiple conditions with if and else bindings by using logical operators, such as && (and), || (or), and ! (not), in your expressions. For example, you can use an if binding to show a button only if the user is logged in and has permission, or an else binding to show a warning message otherwise. You can also use parentheses to group your conditions and control the order of evaluation. For example:

<div data-bind="if: isLoggedIn && hasPermission">

<button data-bind="click: doSomething">Do something</button>

</div>

<div data-bind="else">

<p>You need to log in and have permission to do something.</p>

</div>

You can also use ternary operators, such as ? (then) and : (else), to create shorter expressions with if and else bindings. For example:

<div data-bind="text: status ? 'Online' : 'Offline'"></div>

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?