Angular 9 compiler

15 minute read

Why does Angular have a compiler at all? Main job of the compiler is to turn the template you write into the code that runs the Angular runtime.

Alex Rickabaugh - Angular Connect 2019

In Angular, the developer write templates declaratively that is what to render, the binding, etc. but not how it happens at runtime, there is not description about how the change detection mechanism works. The Angular runtime does not understand the declarative template syntax so it has to be translated into something the runtime can run into a browser.

The compiler takes the declarative Angular syntax and turns it into imperative code.

Why, again, a compiler is required? not possible to do by hand?

  • Lot of boilerplate code, better to focus on the business logic than how things work under the hood.
  • Angular team can optimize and improve by release w.r.t. browser evolution the imperative code.


  • Angular Compiler: part of the Angular rendering architecture that transforms templates and decorators into something that the Angular runtime can understand.
  • Angular Runtime: part of the Angular rendering architecture that runs an Angular application.
  • Angular template declarative syntax: describes what the view has to look like, what the view has to display, but not how.
  • Angular application imperative code: describes how the view has to be rendered via a sequence of JavaScript instructions/commands.
  • Metadata: set of data enclosed into a decorator that describes the entity represented by the decorator itself. For instance a component has the selector, the template, etc. Metadata are then reused by the compiler and put into the definition files .d.ts, the component API. Basically the metadata help to preserve the information removed from .js files.

Quick overview

The work done on the new Angular compiler can be divided in three categories as stated in the implementation status:

  • @angular/compiler-cli: TypeScript transformer pipeline which includes two CLI tools
    • ngtsc: the Angular TypeScript compiler which looks for the Angular decorators like @Component and substitute them with their specific Angular Runtime instructions/counterparts like ɵɵdefineComponent.
    • ngcc: the Angular Compatibility Compiler which converts pre-ivy modules into ivy-module, can be even run as part of a code loader like Webpack to have packages from node_modules transpiled on-the-fly.
  • @angular/compiler: Ivy Compiler which converts decorators into Ivy.
  • @angular/core: decorators which can be converted by the @angular/compiler.

Angular Ivy compiler model

The Ivy model foresees to compile the Angular decorators like @Injectable, @Component, etc into static properties.

All the decorators do not need global knowledge of the application, except @Component which requires information coming from @NgModule. In the module other selectors used by the component template are declared and so the transitive closure of the exports of that module imports.

Without those information the component def (ɵcmp) cannot be correctly generated.

Consider the Welcome to Angular! application:

  selector: 'app-root',
  template: `
    <div style="text-align:center">
        Welcome to {{ title }}!
  styleUrls: []
export class AppComponent {
  @Input() title = 'Angular';

The previous code is characterized by decorators like @Component and @Input, part of the Angular template declarative syntax that provides developers with an easy template grammar to both

  • write control flow statements, like the for loop and if conditional statement,
  • and make data binding between the variables declared in the controller and used in the templates.

Tip For most of the developers the binding is just the use of the same variable name between the template and the controller, but there is a mechanism that manages the change detection at runtime. This is a great thing and the Angular compiler automatically adds this mechanism. Less boilerplate code, templates are mode readable and less error prone.

The Angular runtime is a collection of JavaScript instructions/functions able to render a component template into the DOM and to answer to change detection when something in the model has changed (MVC). Everything must be in JavaScript, ready to be run, so the Angular declarative syntax is translated into these instructions.

To make it more clear try to develop a standard Web Component: when the developer has to deal with the template he must deal with the DOM API to create an element and attach it to the DOM, write some code to detect changes in the model and update the view. In the Angular code there is not trace of these operations since a template is translated into JavaScript imperative code, a series of JavaScript instructions, part of the Angular runtime, that, when invoked, creates the component in the browser.

Most of the tedious and repetitive job is done by the Angular compiler in conjunction with the Angular runtime.

Compiler enables decoupling

The developer just writes the Angular modules and components, that is the what, via the Angular declarative syntax but it does not know/care about the how, that is how things are executed at runtime: compiler enables to decouple the what from the how.

This approach has multiple advantages:

  • minimize or eliminate side effects making the developer life simpler;
  • if the Angular rendering architecture evolves, small or null changes are required to use the latest version;
  • browsers change more and more often as the EcmaScript hence the how the templates are rendered can change accordingly optimizing web performance for instance;
  • templates and decorators can be compiled differently w.r.t. the platform where the code is run such ES5 or ES6 with module support.

Simple project setup

Angular 9 has been recently released, generate a simple project with Angular 9 running:

npm install -g @angular/cli     // to install the Angular CLI
ng new angular-nine-ivy         // or the name you want

In case you already have a workspace with some changes you have to commit or stash them, otherwise add the flag --allow-dirty.

If you already have the old Angular CLI, it seems the better way is to uninstall the old one and the install the new globally.

Let’s compile

There are two compiler entry points in the @angular/compiler-cli: ngtsc and ngcc.


It is the TypeScript to JavaScript transpiler that reifies the Angular decorators into static properties. ngc works like the ngtsc when Ivy is enabled.


It is the Angular Compatibility Compiler that process the NPM library code from the node_modules folder producing equivalent library code Ivy compatible. ngcc can also be run by a code loader like Webpack to get packages transpiled on demand instead of written in the node_module package folder.


Open the package.json and add the ngc script:

"scripts": {
    "ng": "ng",
    "ngc": "ngc"

In the tsconfig.json set "declaration": true, in order to have the .d.ts files as well, then run:

$ npm run ngc

The result of the component compilation is located at dist\out-tsc\src. The Welcome to Angular! component is translated into:

import { Component, Input } from '@angular/core';
import * as i0 from "@angular/core";
export class AppComponent {
    constructor() {
        this.title = 'Angular';
AppComponent.ɵfac = function AppComponent_Factory(t) { return new (t || AppComponent)(); };
AppComponent.ɵcmp = i0.ɵɵdefineComponent({ type: AppComponent, selectors: [["app-root"]], inputs: { title: "title" }, decls: 3, vars: 1, consts: [[2, "text-align", "center"]], template: function AppComponent_Template(rf, ctx) { if (rf & 1) {
        i0.ɵɵelementStart(0, "div", 0);
        i0.ɵɵelementStart(1, "h1");
    } if (rf & 2) {
        i0.ɵɵtextInterpolate1(" Welcome to ", ctx.title, " ");
    } }, styles: [""] });
/*@__PURE__*/ (function () { i0.ɵsetClassMetadata(AppComponent, [{
        type: Component,
        args: [{
                selector: 'app-root',
                template: `
    <div style="text-align:center">
        Welcome to 
                styleUrls: ['./app.component.css']
    }], null, { title: [{
            type: Input
        }] }); })();

and then the app.component.d.ts definition file:

import * as i0 from "@angular/core";
export declare class AppComponent {
    title: string;
    static ɵfac: i0.ɵɵFactoryDef<AppComponent>;
    static ɵcmp: i0.ɵɵComponentDefWithMeta<AppComponent, "app-root", never, { "title": "title"; }, {}, never>;

Attention By default Ivy is enabled, if you disable it you will get a different compilation output.

JIT vs. AoT

Compiler can work in JiT (Just in Time) mode, it is delivered along with the application and compiles at runtime. AoT (Ahead of Time) compilation instead compiles everything at build time making the application faster and it does not required to ship the compiler with the application.

With Angular 9, thanks to Ivy, the compilation is faster and by default "aot": true.

Compilation models

Both TypeScript and Angular Compiler preserve important metadata that cannot be part of the emitted .js files. Angular enhances the .d.ts files with framework specific metadata to be reused for better component type checking.

TypeScript compilation model

JavaScript code has no type information, it is just ready to be executed in the browser. The TypeScript compiler has transformed all the .ts source files but typing information are not totally lost, the definition files help to preserve the interface for future use.

For instance the file library.ts is compiled into library.js and library.d.ts is produced, it describes the interface or public API of the library.

The definition file brings type information to help TypeScript to static type check the usage of the library. Fo example, when an application declare a npm dependency library can then import functionality:

import {AwesomeLib} from 'awesome-lib';
// use the lib

TypeScript will static type check the use of the library in the app.js exploiting the library.d.ts definition file:

declare class AwesomeLib {
    awesomeMethod(): string;

Angular Ivy compilation model

An Angular component has some useful information declared in the decorator like the selector. These information are really important to use the component elsewhere and static type check the code.

The Angular Ivy compiler has been improved to not waste this information and to enrich the definition file public interface. For instance, the following Angular component:

    selector: 'awesome-comp'
export class AwesomeComponent {
    @Input() value: string;

will be compiled into JavaScript and the metadata will enrich the definition file as following:

export declare class AwesomeComponent() {
    value: string;
    static ngComponentDef: ng.ComponentDef<
        {value: 'value'}

The definition file became the component public API. The compiler can exploit it in order to type check the code that will use it:

<awesome-comp [value]="a value">
    just a component

TypeScript transpiler architecture

The following diagram, thanks to Angular team, shows the normal tsc flow and the steps to transpile a .ts file into the .js one.

                           |----------------------------------> | TypeScript |
                           |                                    |   .d.ts    |
                           |                                    |------------|
|------------|          |-----|               |-----|           |------------|
| TypeScript | -parse-> | AST | ->transform-> | AST | ->print-> | JavaScript |
|   source   |    |     |-----|       |       |-----|           |   source   |
|------------|    |        |          |                         |------------|
                  |    type-check     |
                  |        |          |
                  |        v          |
                  |    |--------|     |
                  |--> | errors | <---|
  • parse: recursive descent parser that produces the abstract syntax tree (AST);
  • type check: perform type analysis on every file, report found error. Not modified by the ngtsc;
  • AST to AST: remove type declarations, convert class into ES5, async methods, etc.

Extension points

TypeScript tsc provides some extension points to alter its output:

  • CompilerHost.getSourceFile to modify the source;
  • CustomTransformers to alter the list of transforms;
  • WriteFileCallback to intercept the output before it is written.

Compilation steps

The ngtsc is a wrapper around the tsc, the TypeScript compiler, that extends and modify the normal compilation flow.


Tip The TypeScript transpiler cannot compile the Angular templates and decorators, so the Angular compiler kicks in to reify Angular decorators into static properties. Once finished the TypeScript compiler can go on producing JavaScript code. In other words the Angular compiler allows the code written in Angular declarative syntax to participate to the TypeScript compilation process.

1. Program creation

Starting from the tsconfig.json file, the TypeScript process discover the application source files via the import statements.

2. Analysis

The Angular compiler takes all the .ts files collected and, class by class, looks for Angular declarative syntax code, basically Angular things. The compiler gathers isolation information about components for instance, but not about modules. Remember, for the @Component the compiler requires a global knowledge about the module due transitive closure resolution of the exports of the component module template imports used in the component template.

3. Resolve

The Angular compiler looks again at the whole application but this time in a larger picture including modules as well. All the code now is understandable and parsable by the next step of the TypeScript compiler. Optimizations will take place at this step.

4. Type checking

TypeScript checks error in the application, templates included, that now are a series of imperative instructions.

5. Emit

The most expensive operation in the compilation process, the TypeScript code is transformed into JavaScript ready to be run by the browser. Angular component classes have now only imperative code to describe what a template looks like.

Compiler features

Angular compiler has many interesting features, some of them have been enhanced and improved with the new Angular Ivy architecture. Let’s see some of them.

Module compilation scope

The module scope allows the compiler to resolve uniquely the Angular components used by the application. Consider the following application module:

    declarations: [
export class AppModule {}

The module decorator property declarations is the module compilation scope, it holds an array of Angular components used in the application templates. The HelloComponent is a component coming from a library with its own definition file enriched with metadata as seen before.

The developer declares the willingness to use the component in the array, then can use the component adding the corresponding selector in one of the application templates.

The compiler can than uniquely match the component, verify the use of the selectors along with its attributes.

Export compilation scope

A module can be used to export Angular components as a method to make visible the external world some implemented components.

Tip The module pattern is quite frequently used in computer languages as a way to separate a complex application in small chunks that can be reused elsewhere. Just think about ESM or CommonJS etc. Angular provides a way to create a modular application via the module concept as well. A module is used also to hide implementation details and to select what should be public. The exports property is the way chosen to make Angular component public so re-usable by the rest of the world.

In the following example the HelloModule both declares and exports the HelloWorld component. The module library just implements one component and exports it to all the applications that want to use it:

    declaration: [HelloComponent],
    exports: [HelloComponent]
export class HelloModule {}

When an application wants to use the HelloWorld component has just to declare it:

    declarations: [
export class AppModule {}

The compiler then can:

  • uniquely find the component definition;
  • figure out from the template which components are used;
  • generate the code and reference it accordingly;
  • help the tree-shaker to remove things that are not referenced.

Partial evaluation

Compiler actually attempt to almost run TypeScript code in decorators

Alex Rickabaugh - Angular Connect 2019

Angular compiler contains almost a complete TypeScript interpreter. In the following example:

import {SOME_MODULES} from './some_module';

    declarations: [HelloComponent, ByeComponent],
    exports: [HelloComponent, ByeComponent],
    imports: [...SOME_MODULES]
export class AnotherModuel {...}

the compiler can follow and evaluate the import references. Some evaluation are just partial since there are not enough information at compiler time.

For instance:

import {SOME_MODULES} from './some_module';

    imports: [SOME_MODULES.modules]

export const SOME_MODULES = {
    modules: [HelloModule, ByeModule],

    // not available at compile time
    viewportSize: {
        x: document.body.scrollWidth,
        y: document.body.scrollHeight

only at runtime some values will available. This value are dynamic expressions. The compiler sees the previous object more or less like this:

    "modules": [
    "viewportSize": {
        x: DynamicValue(document.body.scrollWidth),
        y: DynamicValue(document.body.scrollHeight)


  • SOME_MODULES is an object with two properties;
  • DynamicValue is an indicator to say “cannot get past”.

The compilation does not stop, it goes on since a situation like this could be quite common.

import {SOME_MODULES} from './some_module'
    styles: [`
        :host {
            width: $SOME_MODULES.viewportSize.x

While the compilation goes on for modules properties, it cannot for viewportSize since the value cannot be figured out. An explicative error message is produced about styles.

Template type checking

A template and expressions

    [account]="getAccount(, 'primary') | async">

they are compiled into TypeScript code becoming type check blocks. So blocks are sent to TypeScript compiler and possible errors are then return in the context of the template.

function typeCheckBlock(ctx: AppComponent) {
    let cmp!: AccountViewCmp;
    let pipe!: ng.AsyncPipe;

    cmp.account /*273,356*/ = (pipe.transform(
        ctx.getAccount((ctx.user /*311,315*/).id /*311,318*/,
        "primary" /*320,329*/)
    /*300,338*/) /*289,339*/;

How is it possible to return an error in the context of a template? Well, code is translated with additional offset comments that allows then the contextualization.


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

  selector: 'app-root',
  template: `
    <div style="text-align:center">
        Welcome to {{ title }}!

        <tr *ngFor="let hero of heroes">
  styleUrls: []
export class AppComponent {
  @Input() title = 'Angular';

  heroes = 'fake array';

Angular version 9 with Ivy enabled maintains the behavior of the fullTemplateTypeCheck flag and introduces a strict mode with the flag strictTemplates that goes beyond the Angular 8 type checker. Activate the strict mode in the tsconfig.json:

"angularCompilerOptions": {
    "fullTemplateTypeCheck": true,
    "strictTemplates": true,

Re-run the npm run ngc and get a full better error reported by the compiler:

src/app/app.component.html:7:13 - error NG2339: Property 'name' does not exist on type 'string'.

7       <td>{{}}</td>

    5   templateUrl: './app.component.html',
    Error occurs in the template of component AppComponent.

More details available on the template type-check page.


The Angular 9 Ivy rendering architecture introduces a new compiler and a new rendering engine not only to exploit the incremental DOM technique but also a more powerful compiler.

Ivy is enabled by default, --aot is the default way of developing since the new compiler is faster than the previous one. Having AoT mode enable by default reduces also the risk of discrepancies between development and production code.

Ivy compiler goes even further, it has a better type-check making the reported error much more explicative and easier for the developer to identify the root cause of the issue.

We all are looking forward to see the new compiler applied to Angular Elements.


Angular Compiler Architecture