Svelte Tutorial - 9 - Conditional Rendering

Summary

This video tutorial explains how to conditionally render HTML elements in Svelte applications using 'if', 'else', and 'else if' blocks. It demonstrates with an example where a variable's value determines the displayed message, such as 'zero', 'positive', 'negative', or 'not a number'. The tutorial walks through the syntax and usage of these blocks, showing how to update the UI based on script-defined data, making it simple to handle multiple conditions with clear and practical examples.

Takeaways

• 😀 Building web applications with Svelte involves using special HTML blocks to conditionally render elements based on certain conditions.
• 🔑 Svelte provides three special blocks: `if`, `else`, and `elsif`, which function similarly to traditional programming language constructs.
• 📝 The `if` block in Svelte uses a syntax of `{#if ...}` to evaluate an expression and render HTML if the expression is true.
• 📚 The `else` block is used to render alternative content when the `if` block condition is false, utilizing `{:else ...}` syntax.
• 🔄 The `elsif` block, represented by `{:else if ...}`, allows for additional conditions to be checked after the initial `if` block.
• 🎯 An example given in the script involves checking a variable to determine if it's zero, positive, negative, or not a number, and rendering HTML accordingly.
• 📌 The `#` symbol within curly braces denotes the start of an `if` block, while the `/` symbol signifies the end of the block.
• 🔍 Conditional rendering in Svelte is demonstrated by changing the value of a variable and observing the changes in the browser output.
• 📈 The script explains how to progressively build up conditional statements by adding `else if` blocks for multiple conditions.
• 🛠️ The final example in the script shows how to handle various conditions by using a combination of `if`, `else`, and `elsif` blocks to render different HTML elements.
• 📺 The video script concludes with a demonstration of the conditional rendering working as expected with different values assigned to the variable.

Q & A

What are the three special HTML blocks provided by Swelt for conditional rendering?

The three special HTML blocks provided by Swelt are the 'if' block, 'else' block, and 'elsif' block.

How do the conditional blocks in Swelt differ from traditional programming language constructs?

The conditional blocks in Swelt differ in that they are used to render HTML elements rather than execute logic.

What is the syntax for starting an 'if' block in Swelt?

The syntax for starting an 'if' block in Swelt is '{#if expression}'.

How do you close an 'if' block in Swelt?

An 'if' block in Swelt is closed with '}}' followed by the 'if' keyword.

What is the purpose of the 'else' block in Swelt's conditional rendering?

The 'else' block in Swelt is used to render HTML elements when the 'if' condition evaluates to false.

What symbol is used to represent the continuation of an 'else' block in Swelt?

The continuation of an 'else' block in Swelt is represented by a colon ':'.

How is the 'elsif' block used in Swelt to handle multiple conditions?

The 'elsif' block is used after the 'if' block to check for additional conditions, using the syntax '{:else if condition}'.

What happens when none of the conditions in the 'if', 'else if', and 'else' blocks are met?

When none of the conditions are met, no HTML elements are rendered, unless specified otherwise in the script.

Can you provide an example of how to check if a number is negative in Swelt?

To check if a number is negative in Swelt, you would use an 'else if' block with the condition '{:else if num < 0}'.

How can you display different messages based on whether a number is zero, positive, negative, or not a number in Swelt?

You can use a combination of 'if', 'else if', and 'else' blocks to check the conditions and render the appropriate message in Swelt.

What does the 'hash f' within curly braces represent in Swelt's conditional blocks?

The 'hash f' within curly braces in Swelt represents the opening of the conditional block where the expression is evaluated.

Outlines

00:00

📝 Conditional Rendering in Svelte

This paragraph explains how to use Svelte's conditional rendering features to display HTML elements based on certain conditions. It introduces the 'if', 'else', and 'elsif' blocks, which function similarly to traditional programming language constructs but are used to render HTML instead of executing logic. The explanation is illustrated with an example where a variable's value determines the displayed message. The process includes defining an 'if' block to check if a number is zero, using an 'else' block to handle non-zero cases, and adding 'else if' blocks for additional conditions such as checking for negative or positive numbers. The paragraph concludes with a demonstration of how to update the code to display messages for different conditions and emphasizes the simplicity of conditional rendering in Svelte.

05:01

🔧 Testing Conditional Rendering in Svelte

In this paragraph, the focus is on testing the conditional rendering logic implemented in Svelte. The script starts with a predefined variable set to a positive number, demonstrating the rendering of a positive number message. The author then modifies the variable to a negative number to show how the UI updates accordingly. Further, assigning a string to the variable correctly triggers the 'not a number' condition, validating the robustness of the conditional rendering. The paragraph emphasizes the successful functioning of the conditional rendering and encourages viewers to subscribe for more content, signaling the end of the tutorial.

Keywords

💡Svelte

Svelte is a modern front-end JavaScript framework used for building user interfaces. It allows developers to write components with minimal boilerplate code. In the video, Svelte is introduced as the framework being used to conditionally render HTML elements based on specific conditions.

💡If Block

An If Block in Svelte is a conditional statement used to render HTML elements only if a certain condition is true. The video explains that the If Block is defined using curly braces and the 'if' keyword, and it is used to check whether a number is zero in the provided example.

💡Else Block

The Else Block in Svelte is used in conjunction with an If Block to provide an alternative rendering when the initial condition is false. In the video, the Else Block is introduced to display a message when the number is not zero, offering a fallback content option.

💡Else If Block

The Else If Block is used to check additional conditions if the initial If Block condition is false. This allows for multiple conditional checks in a sequence. The video demonstrates its use by checking whether a number is positive or negative after confirming it is not zero.

💡Conditional Rendering

Conditional rendering refers to the practice of displaying content in the user interface based on specific conditions. The video shows how Svelte makes conditional rendering simple by using If, Else, and Else If Blocks to control which HTML elements are displayed depending on the value of a variable.

💡HTML Elements

HTML elements are the building blocks of web pages, used to structure and display content. In the context of the video, HTML elements like <h2> tags are conditionally rendered using Svelte’s If, Else, and Else If Blocks to display different messages based on the value of a number.

💡JavaScript Expression

A JavaScript expression is any valid piece of code that evaluates to a value. In the video, expressions like 'num === 0' are used within Svelte's conditional blocks to determine which HTML elements should be rendered.

💡Script Section

The script section in Svelte is where JavaScript code is written to define variables and logic that interact with the HTML template. The video shows how a variable, 'num', is defined in the script section and then used in the If, Else, and Else If Blocks to conditionally render content.

💡Rendering

Rendering in web development refers to the process of generating the visual output of a web page or component. The video focuses on conditional rendering, where the HTML output changes dynamically based on the logic defined in Svelte's If, Else, and Else If Blocks.

💡Not a Number (NaN)

NaN, or 'Not a Number', is a special value in JavaScript that represents an invalid number. The video uses NaN as one of the conditions to demonstrate how Svelte can handle different types of input, displaying a message if the value is not a valid number.

Highlights

Swelt applications allow conditional rendering of HTML elements based on certain conditions.

Swelt provides special HTML blocks: if block, else block, and elsif block for conditional rendering.

These blocks function similarly to if-else and else-if statements in programming languages but render HTML instead of executing logic.

An example scenario is given to demonstrate conditional rendering based on the value of a variable.

A constant 'num' is defined and used to display different messages based on whether it is zero, positive, negative, or not a number.

The if block syntax in Swelt is demonstrated with curly braces, a hash symbol, and the 'if' keyword followed by the expression to evaluate.

The if block is closed with a pair of curly braces containing a forward slash and the 'if' keyword.

Changing the value of 'num' to 5 demonstrates the if block rendering as false and the heading not appearing in the browser.

The else block is introduced to display content when the if condition is not met, using a continuation block with a colon and the 'else' keyword.

Conditional rendering is shown to display different headings based on whether the number is zero or not.

The else if block is explained for handling multiple conditions, with its syntax demonstrated using curly braces, a colon, 'else', a space, and 'if'.

Additional conditional elements are added to display messages for zero, negative, positive, or non-number values.

The process of copying and modifying the else if block for different conditions is described.

A final check is added to display a message if none of the conditions are met, indicating the number is not a number.

The code is saved and verified to work as expected, with different values for 'num' showing the correct conditional rendering in the browser.

The if-else and else-if logical blocks in Swelt are summarized as tools for conditionally rendering HTML elements in the UI.

The video concludes with a thank you and an invitation to subscribe for more content.

Transcripts

when you're building swelt applications

you may often need to show or hide some

html

based on a certain condition swelt

makes it really simple to do that by

providing

three special html blocks

they are if block else block and

the elsif block the three blocks work

similar to any other programming

language where we use

if else and else if statements

the only difference here is that we

render html elements

rather than execute some logic let's

understand how they work with an example

let's consider a very simple scenario

given a variable we need to display if

it is

0 a positive number a negative number

or not a number let's begin

by defining a new constant const

num is equal to zero

first let's check if the number is zero

or not we begin by adding an if block

a syntax is curly braces then

the hash symbol followed by

if keyword this

is followed by the expression we want to

evaluate

our expression is num is equal to zero

if it evaluates to true we need to

output

some text so let's add an h2 tag

with the text the number

is zero the hash

f within curly braces represents the

opening

of the if block we close this if block

by adding another pair of curly braces

but this time a forward slash

followed by the if keyword

if we now save the file and take a look

at the browser

we should see the text the number is

zero

if i change the number to 5

the expression evaluates to false

and the heading is not rendered in the

browser

fairly simple as you can see but let's

now

improve this instead of not displaying

anything let's display that the number

is

not zero and for that we make use of

the else block right after

the h2 tag we are going to add another

block

represented by curly braces

but this block is not the start or the

end

it is a continuation block and is

represented

by a colon this is followed by the else

keyword within the else block we add

another heading that says the number is

not

0. so h2 tag

the number is not 0.

if we save the file and take a look at

the browser

the text now reads the number is not 0.

we are conditionally rendering our html

based on

data defined in the script section

if the number is zero we display the

first

heading else we display the second

heading

now when you just have an either or

condition the if else blocks are

sufficient

but if you have more number of

conditions then you have to make use of

the else if block as well

let's add a few more conditional html

elements

let's display if the number is zero or

negative

or positive or not a number

now we are already checking if the

number is zero

next let's check if the number is

negative

or positive so right after

the first h2 element we add the else if

block

the syntax is curly braces colon

else space if and this

is followed by the condition the

condition

is that num is less than

zero now if this condition is true

we render another heading that says the

number

is negative make a copy

of the two lines paste it change

condition is num greater than zero

and the text negative to positive

so if the number is greater than zero we

output the text

the number is positive finally

if none of the conditions are satisfied

we display

that it is not a number

let's save the file and verify if this

code works

right now num is set to 5

and hence we see that it is positive

let me change it to minus 5

and you can see that the number is now

negative

if i assign a string

you can see that it is not a number

our conditional rendering markup is

working as expected

all right that is about the if else

and else if logical blocks in swelled

they are used to conditionally render

html elements in the ui

thank you guys for watching make sure to

subscribe to the channel and i'll see

you guys in the next one