ngOnChanges for input changes in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an @Input property changes in Angular?

Consider a child component with an @Input property. When the parent changes the input value, what does ngOnChanges receive?

Angular

import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-child',
  template: `<p>{{data}}</p>`
})
export class ChildComponent implements OnChanges {
  @Input() data!: string;

  ngOnChanges(changes: SimpleChanges) {
    console.log(changes);
  }
}

A An object with keys for each changed input, each containing previousValue and currentValue.

B A string with the new input value only.

C An array of all input property names.

D No argument is passed to ngOnChanges.

💡 Hint

Think about how Angular tracks changes to inputs and what information it provides.

The ngOnChanges method receives a SimpleChanges object. This object has keys for each input property that changed. Each key holds an object with previousValue, currentValue, and firstChange properties.

❓ state_output

intermediate

What is logged when the input changes twice?

Given this Angular component, what will be logged to the console after the parent changes the input from 'one' to 'two' and then to 'three'?

Angular

import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-child',
  template: `<p>{{value}}</p>`
})
export class ChildComponent implements OnChanges {
  @Input() value!: string;

  ngOnChanges(changes: SimpleChanges) {
    console.log(`Changed from ${changes['value'].previousValue} to ${changes['value'].currentValue}`);
  }
}

A

Changed from undefined to one
Changed from one to two

B

Changed from one to two
Changed from two to three

C

Changed from undefined to one
Changed from undefined to two
Changed from undefined to three

D

Changed from undefined to one
Changed from one to two
Changed from two to three

💡 Hint

Remember the first change has previousValue as undefined.

On the first input change (initial binding), previousValue is undefined. Subsequent changes show the last value as previousValue. The prompt describes subsequent changes starting from 'one', so the logs are 'Changed from one to two' followed by 'Changed from two to three'.

📝 Syntax

advanced

Which code snippet correctly implements ngOnChanges to detect input changes?

Choose the Angular component code that correctly implements ngOnChanges to detect changes on an input property count.

A

ngOnChanges(changes: SimpleChanges) {
  console.log(changes.currentValue);
}

B

ngOnChanges() {
  console.log('Count changed:', this.count);
}

C

ngOnChanges(changes: SimpleChanges) {
  if (changes.count) {
    console.log('Count changed:', changes.count.currentValue);
  }
}

D

ngOnChanges(changes: any) {
  console.log(changes.count.previousValue);
}

💡 Hint

Check how SimpleChanges is used to access changed inputs.

Option C correctly checks if the count property changed and accesses its currentValue. Option C lacks the changes parameter and cannot detect previous values. Option C uses any but does not check if count exists. Option C tries to access changes.currentValue which is invalid.

🔧 Debug

advanced

Why does ngOnChanges not fire when an input object's property changes?

Given this parent and child component, why does ngOnChanges in the child not trigger when a property inside the input object changes?

Parent template:
<app-child [config]="config"></app-child>

Parent component:
config = { theme: 'dark' };

// Later update
this.config.theme = 'light';

A Because the input property was not marked with @Input decorator.

B Because ngOnChanges only works with primitive inputs, not objects.

C Because the child component must implement ngDoCheck instead of ngOnChanges.

D Because Angular compares input references, not deep object properties, so the reference did not change.

💡 Hint

Think about how Angular detects changes for objects passed as inputs.

Angular's change detection for inputs uses reference checks. If the object reference does not change, ngOnChanges does not fire, even if internal properties change. To detect such changes, you must replace the object with a new reference or use other lifecycle hooks.

🧠 Conceptual

expert

What is the correct sequence of Angular lifecycle hooks when an input changes?

Order the lifecycle hooks in the sequence Angular calls them when an input property changes on a component.

A 2, 1, 3, 4

B 1, 2, 3, 4

C 3, 2, 1, 4

D 2, 3, 1, 4

💡 Hint

Remember which hooks run before and after initialization and which run on every change detection cycle.

When an input changes, Angular calls ngOnChanges first, then ngOnInit (only once after first change), then ngDoCheck on every change detection, and finally ngAfterViewInit after the component's view is initialized.