ngOnChanges for input changes in Angular - Step-by-Step Execution

Concept Flow - ngOnChanges for input changes

Parent changes input value

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Angular detects input change

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Calls ngOnChanges() in child

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ngOnChanges receives SimpleChanges object

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Child updates based on changes

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View updates with new input

When a parent changes an input property, Angular calls ngOnChanges in the child with details of the change, allowing the child to react before the view updates.

Execution Sample

Angular

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

@Component({ selector: 'child-comp', template: '{{data}}' })
export class ChildComponent implements OnChanges {
  @Input() data = '';
  ngOnChanges(changes: SimpleChanges) {
    console.log('Data changed:', changes['data'].currentValue);
  }
}

This child component logs changes to its input property 'data' whenever the parent updates it.

Execution Table

Step	Parent changes input	ngOnChanges called	SimpleChanges content	Child action	View update
1	data = 'Hello'	Yes	{ data: { previousValue: undefined, currentValue: 'Hello', firstChange: true } }	Logs: Data changed: Hello	View shows 'Hello'
2	data = 'World'	Yes	{ data: { previousValue: 'Hello', currentValue: 'World', firstChange: false } }	Logs: Data changed: World	View shows 'World'
3	data = 'World'	No	No change detected	No log	View remains 'World'
4	data = 'Angular'	Yes	{ data: { previousValue: 'World', currentValue: 'Angular', firstChange: false } }	Logs: Data changed: Angular	View shows 'Angular'

💡 No further input changes, ngOnChanges not called.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4
data	'' (empty)	'Hello'	'World'	'World'	'Angular'

Key Moments - 3 Insights

Why is ngOnChanges not called when the input value stays the same?

Because Angular only calls ngOnChanges when it detects a change in the input value. Step 3 in the execution_table shows no call since 'data' remains 'World'.

What does the SimpleChanges object contain?

It contains previousValue, currentValue, and firstChange for each changed input. See steps 1, 2, and 4 in the execution_table for examples.

Can ngOnChanges detect multiple input changes at once?

Yes, ngOnChanges receives all changed inputs in the SimpleChanges object. This example shows one input, but Angular supports multiple inputs.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'data' after step 2?

A 'Hello'

B 'World'

C 'Angular'

D '' (empty string)

💡 Hint

Check the 'data' variable in variable_tracker after 'After 2'

At which step does ngOnChanges NOT get called?

A Step 1

B Step 4

C Step 3

D Step 2

💡 Hint

Look at the 'ngOnChanges called' column in execution_table

If the parent changes 'data' to the same value twice in a row, what happens?

A ngOnChanges is not called the second time

B ngOnChanges is called once

C ngOnChanges is called twice

D The view does not update at all

💡 Hint

Refer to step 3 in execution_table where input stays the same

Concept Snapshot

ngOnChanges is a lifecycle hook called when Angular detects input property changes.
It receives a SimpleChanges object with previous and current values.
Use it to react to input changes before the view updates.
Called only when input values actually change.
Useful for initialization or side effects on input update.

Full Transcript

This visual execution shows how Angular calls ngOnChanges in a child component when its input property changes. When the parent updates the input, Angular detects the change and calls ngOnChanges with a SimpleChanges object describing the old and new values. The child can then react, such as logging the new value or updating internal state. If the input value does not change, ngOnChanges is not called. This helps components respond only to real changes. The view updates after ngOnChanges runs, showing the latest input value.