Pure vs impure pipes in Angular - When to Use Which

The Big Idea

Discover how a simple choice in pipes can make your Angular app faster and smoother!

The Scenario

Imagine you have a list of items that updates often, and you want to transform or filter this list in your Angular template manually every time the data changes.

The Problem

Manually updating or recalculating transformations on every change is slow and error-prone. It can cause unnecessary work and make your app lag or behave unpredictably.

The Solution

Pure and impure pipes let Angular handle these transformations efficiently. Pure pipes run only when inputs change, while impure pipes run on every change detection, giving you control over performance and updates.

Before vs After

✗ Before

filteredList = filterItems(items); // called manually on every change

✓ After

<div *ngFor="let item of items | customPipe">{{ item }}</div>

What It Enables

This lets you write clean templates that update automatically and efficiently without extra manual work.

Real Life Example

Showing a filtered product list that updates only when the product data changes, not on every user interaction or timer tick.

Key Takeaways

Pure pipes run only when inputs change, improving performance.

Impure pipes run on every change detection, useful for dynamic data.

Choosing the right pipe type helps balance app speed and responsiveness.

Practice

1. What is the main difference between a pure pipe and an impure pipe in Angular?

easy

A. Pure pipes can modify input data, impure pipes cannot.

B. Pure pipes run every change detection cycle, impure pipes run only when input references change.

C. Pure pipes run only when input references change, impure pipes run every change detection cycle.

D. Pure pipes are slower than impure pipes.

Solution

1. Step 1: Understand pure pipe behavior

   Pure pipes execute only when the input reference changes, meaning they do not run on every change detection cycle.

2. Step 2: Understand impure pipe behavior

   Impure pipes run on every change detection cycle regardless of input changes, which can impact performance.

3. Final Answer:

   Pure pipes run only when input references change, impure pipes run every change detection cycle. -> Option C

4. Quick Check:

   Pure vs impure pipe execution = A [OK]

Hint: Pure pipes run less often; impure pipes run every cycle [OK]

Common Mistakes:

• Confusing when pure and impure pipes run
• Thinking pure pipes run every cycle
• Assuming impure pipes only run on input change

2. Which of the following is the correct way to declare an impure pipe in Angular?

easy

A. @Pipe({ name: 'examplePipe', pure: false })

B. @Pipe({ name: 'examplePipe', impure: true })

C. @Pipe({ name: 'examplePipe' })

D. @Pipe({ name: 'examplePipe', pure: true })

Solution

1. Step 1: Recall pipe metadata syntax

   Angular pipes use the @Pipe decorator with a 'pure' property to indicate purity.

2. Step 2: Identify impure pipe declaration

   Setting 'pure: false' marks the pipe as impure, causing it to run every change detection cycle.

3. Final Answer:

   @Pipe({ name: 'examplePipe', pure: false }) -> Option A

4. Quick Check:

   Impure pipe uses pure: false [OK]

Hint: Impure pipes set pure to false in @Pipe decorator [OK]

Common Mistakes:

• Using pure: true for impure pipes
• Omitting pure property defaults to pure
• Using non-existent 'impure' property

3. Given this Angular pipe usage:

items = [1, 2, 3];
// Template: {{ items | customPipe }}

If customPipe is pure and the array items is mutated by items.push(4) without changing the reference, what will happen?

medium

A. The pipe will run and update the output to include 4.

B. Angular throws an error due to mutation.

C. The pipe runs only if change detection is manually triggered.

D. The pipe will not run; output remains unchanged.

Solution

1. Step 1: Understand pure pipe input change detection

   Pure pipes run only when the input reference changes, not when the content inside the object changes.

2. Step 2: Analyze mutation effect on pipe execution

   Mutating the array with push() does not change the reference, so the pure pipe does not run again and output stays the same.

3. Final Answer:

   The pipe will not run; output remains unchanged. -> Option D

4. Quick Check:

   Pure pipe ignores internal mutations = C [OK]

Hint: Pure pipes need new object references to update [OK]

Common Mistakes:

• Assuming mutation triggers pure pipe
• Thinking Angular throws error on mutation
• Believing manual change detection is required here

4. You have an Angular pipe declared as @Pipe({ name: 'filterPipe', pure: true }). It filters an array based on user input. The filtered results do not update when the user types. What is the likely cause?

medium

A. The pipe is impure and runs too often causing errors.

B. The pipe is pure and input array reference does not change on filtering.

C. The pipe is missing the transform method.

D. The pipe is declared without a name.

Solution

1. Step 1: Identify pipe purity and input behavior

   A pure pipe runs only when the input reference changes, not when the content changes.

2. Step 2: Understand why filtered results don't update

   If the array is filtered in place without creating a new array reference, the pure pipe does not detect change and does not update output.

3. Final Answer:

   The pipe is pure and input array reference does not change on filtering. -> Option B

4. Quick Check:

   Pure pipe needs new input reference to update [OK]

Hint: Pure pipes need new input references to update output [OK]

Common Mistakes:

• Assuming impure pipe causes this issue
• Forgetting to return new array reference
• Ignoring pipe naming or method errors

5. You want to create a pipe that formats the current time every second in Angular. Which pipe type should you use and why?

hard

A. Impure pipe, because it runs every change detection cycle and updates time frequently.

B. Pure pipe, because it runs only when input changes, improving performance.

C. Pure pipe, because it automatically triggers updates every second.

D. Impure pipe, because pure pipes cannot be used for formatting.

Solution

1. Step 1: Understand the need for frequent updates

   Formatting current time every second requires the pipe to run frequently to update the display.

2. Step 2: Choose pipe type based on update frequency

   Impure pipes run every change detection cycle, making them suitable for frequent updates like a clock.

3. Final Answer:

   Impure pipe, because it runs every change detection cycle and updates time frequently. -> Option A

4. Quick Check:

   Frequent updates need impure pipe [OK]

Hint: Use impure pipes for frequent, automatic updates [OK]

Common Mistakes:

• Choosing pure pipe for frequent updates
• Thinking pure pipes auto-update every second
• Confusing pipe purpose with formatting capability