Service-to-service injection in Angular - Step-by-Step Execution

Concept Flow - Service-to-service injection

Create ServiceB

↓

Inject ServiceB into ServiceA

↓

ServiceA uses ServiceB's methods

↓

Component injects ServiceA

↓

Component calls ServiceA method -> triggers ServiceB method

ServiceA is created and receives ServiceB inside it. Then a component uses ServiceA, which internally calls ServiceB.

Execution Sample

Angular

import { Injectable } from '@angular/core';

@Injectable({ providedIn: 'root' })
export class ServiceB {
  getValue() { return 'Hello from B'; }
}

@Injectable({ providedIn: 'root' })
export class ServiceA {
  constructor(private serviceB: ServiceB) {}
  getCombined() { return `A + ${this.serviceB.getValue()}`; }
}

ServiceA injects ServiceB and uses its method to return a combined string.

Execution Table

Step	Action	ServiceA Instance	ServiceB Instance	Output
1	Create ServiceB instance	null	ServiceB created	null
2	Create ServiceA instance with ServiceB injected	ServiceA created with ServiceB	ServiceB instance passed	null
3	Component calls ServiceA.getCombined()	ServiceA instance	ServiceB instance	Calls ServiceB.getValue()
4	ServiceB.getValue() returns string	ServiceA instance	ServiceB instance	'Hello from B'
5	ServiceA.getCombined() returns combined string	ServiceA instance	ServiceB instance	'A + Hello from B'
6	Component receives output	ServiceA instance	ServiceB instance	'A + Hello from B'

💡 Execution stops after component receives combined string from ServiceA.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	Final
ServiceA Instance	null	null	created with ServiceB	exists	exists	exists
ServiceB Instance	null	created	passed to ServiceA	exists	exists	exists
Output	null	null	null	null	'A + Hello from B'	'A + Hello from B'

Key Moments - 2 Insights

Why does ServiceA need ServiceB injected in its constructor?

Because Angular injects ServiceB instance into ServiceA automatically, so ServiceA can call ServiceB's methods as shown in execution_table step 2 and 3.

How does the component get the combined string from ServiceA?

The component calls ServiceA.getCombined(), which internally calls ServiceB.getValue(), returning the combined string as shown in execution_table steps 3 to 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 4?

A null

B 'A + Hello from B'

C 'Hello from B'

D 'ServiceB instance'

💡 Hint

Check the Output column at step 4 in the execution_table.

At which step does ServiceA get created with ServiceB injected?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look at the Action column describing ServiceA creation in execution_table.

If ServiceB was not provided in root, what would happen during ServiceA creation?

A ServiceA would create ServiceB itself

B Angular would throw an error about missing provider

C ServiceB would be null but no error

D ServiceA would work normally

💡 Hint

Think about Angular dependency injection rules and what happens if a service is not provided.

Concept Snapshot

Service-to-service injection in Angular:
- Use @Injectable({providedIn: 'root'}) for services
- Inject one service into another via constructor
- Angular provides instances automatically
- Components inject top-level service
- ServiceA can call ServiceB methods internally
- Enables modular, reusable code

Full Transcript

This example shows how Angular injects one service into another. First, ServiceB is created by Angular. Then ServiceA is created with ServiceB injected into its constructor. When a component calls ServiceA's method, ServiceA internally calls ServiceB's method and returns a combined string. The execution table traces each step: creation of ServiceB, creation of ServiceA with ServiceB, method calls, and final output. Variables track the instances and output values. Key moments clarify why constructor injection is needed and how the component receives the combined result. The quiz tests understanding of the injection steps and Angular's provider system. This pattern helps keep code modular and easy to maintain.