Svelte Tutorial - 9 - Conditional Rendering
Summary
TLDRThis 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
📝 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.
🔧 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.
Mindmap
Keywords
💡Svelte
💡If Block
💡Else Block
💡Else If Block
💡Conditional Rendering
💡HTML Elements
💡JavaScript Expression
💡Script Section
💡Rendering
💡Not a Number (NaN)
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
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