Pure vs impure pipes in Angular - Quick Revision & Key Differences

Recall & Review

beginner

What is a pure pipe in Angular?

A pure pipe is a pipe that Angular executes only when it detects a pure change to the input value. Pure changes include changes to primitive types or object references. Pure pipes improve performance by avoiding unnecessary recalculations.

beginner

What is an impure pipe in Angular?

An impure pipe is a pipe that Angular executes on every change detection cycle, regardless of whether the input value changed. This is useful when the pipe depends on mutable objects or external state.

intermediate

How do you declare a pipe as impure in Angular?

You declare a pipe as impure by setting the 'pure' property to false in the @Pipe decorator, like this: @Pipe({name: 'myPipe', pure: false}).

intermediate

Why should you avoid using impure pipes when possible?

Impure pipes run on every change detection cycle, which can hurt performance. Using pure pipes when possible helps Angular skip unnecessary work and keeps apps fast.

advanced

Give an example scenario where an impure pipe is necessary.

When a pipe transforms data from a mutable object like an array that changes internally (e.g., items added or removed without changing the array reference), an impure pipe ensures the view updates correctly.

When does Angular execute a pure pipe?

A When the user clicks a button

B On every change detection cycle

C Only when the input reference or primitive value changes

D Only once when the component loads

Pure pipes run only when Angular detects a pure change to the input, such as a new object reference or a changed primitive value.

How do you mark a pipe as impure in Angular?

A Use @ImpurePipe decorator

B Set pure: false in the @Pipe decorator

C Set pure: true in the @Pipe decorator

D No special setting needed

You mark a pipe as impure by setting pure: false in the @Pipe decorator.

What is a downside of using impure pipes?

A They run on every change detection cycle, which can reduce performance

B They never update the view

C They only work with primitive values

D They require manual triggering

Impure pipes run every change detection cycle, which can slow down the app if overused.

Which pipe type is best for transforming immutable data?

A Pure pipe

B Impure pipe

C Async pipe

D No pipe

Pure pipes are ideal for immutable data because they only run when the input changes.

If you have an array that changes internally but keeps the same reference, which pipe should you use?

A Pure pipe

B No pipe

C Async pipe

D Impure pipe

Impure pipes detect internal changes in mutable objects even if the reference stays the same.

Explain the difference between pure and impure pipes in Angular and when to use each.

Think about how Angular detects changes and when it runs pipes.

Describe how to declare an impure pipe and why you might need one.

Focus on the decorator setting and use case.

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