Introduction to StrictPropertyInitialization in TypeScript

TypeScript is a superset of JavaScript that offers optional static typing, strict null checks, and more control over the type system. However, even with TypeScript’s type system, it is still possible to leave uninitialized object properties, which could lead to runtime errors. To address this issue, TypeScript 2.7 introduced the “strictPropertyInitialization” flag that forces developers to initialize all declared properties or use the question mark (?) to declare optional properties in classes.

In this article, we will explore the benefits and drawbacks of using strictPropertyInitialization in TypeScript and how to overcome the related challenges.

Why use strictPropertyInitialization

The primary reason to use strictPropertyInitialization is to prevent runtime errors due to uninitialized properties. This bug can manifest itself in various ways such as:

  1. TypeError: Cannot read property ‘x’ of undefined
  2. undefined is not an object (evaluating ‘obj.x’)
  3. Uncaught TypeError: Cannot set property ‘x’ of undefined

These errors can be costly to debug, and with modern tools such as webpack and minifiers, these errors can be further obfuscated, making it even more challenging to track down the root cause.

By using strictPropertyInitialization, TypeScript forces developers to initialize all declared properties during the constructor or via default values. If a property is not initialized, then a compile-time error is generated, explicitly identifying the issue and preventing the runtime errors.

The drawbacks of using strictPropertyInitialization

While strictPropertyInitialization offers benefits, it can also impose restrictions on developers. One of the main drawbacks of strictPropertyInitialization is that it requires some level of extra work to declare and initialize the properties correctly. Furthermore, it can impose limitations on the use of optional properties, as developers must declare them with the question mark (?) syntax.

For developers who are not familiar with TypeScript or have a preference for more dynamic languages such as JavaScript, strictPropertyInitialization could be challenging to work with initially. Additionally, projects that rely heavily on third-party libraries could find difficulties in implementing strictPropertyInitialization because not all libraries follow strict initialization rules.

How to use strictPropertyInitialization

To use strictPropertyInitialization, you first need to enable it in your TypeScript configuration file. In tsconfig.json, set the strictPropertyInitialization flag to true.

"compilerOptions": {
"strictPropertyInitialization": true

Next, when creating object classes, ensure all properties are initialized in the constructor or have default values.

class Example {
x: number;
y: number = 0;
constructor() {
this.x = 42;

If a property cannot be initialized in the constructor and has a specific order for initialization, you can use the initializer syntax.

class Example {
x!: number; // Initialized later
y!: number; // Initialized later
constructor() {}
initialize(x: number, y: number) {
this.x = x;
this.y = y;

Finally, when declaring optional properties in the class, use the question mark (?) syntax.

class Example {
x?: number;
constructor() {}
setX(x?: number) {
this.x = x;

The benefits of strictPropertyInitialization in large projects

While strictPropertyInitialization might seem unnecessary in smaller projects, larger projects with multiple developers could benefit from its use. When large-scale codebases get harder to manage, it’s useful to have strict rules in place to ensure code quality and avoid runtime errors caused by uninitialized properties.

By enforcing initialization of all declared properties, strictPropertyInitialization reduces the likelihood of runtime errors in production, simplifying the debugging process. Furthermore, by using the question mark syntax (?), developers can explicitly define optional properties, improving code readability and making it easier for other developers to understand the code’s intention.


What is strictPropertyInitialization in TypeScript?

StrictPropertyInitialization is a TypeScript compiler flag that checks if all declared properties in classes are initialized and throws an error at compile-time if a property is not initialized.

What are the benefits of strictPropertyInitialization?

StrictPropertyInitialization prevents runtime errors caused by uninitialized properties, simplifies debugging, and improves code quality.

What are the drawbacks of strictPropertyInitialization?

StrictPropertyInitialization requires extra work to declare and initialize properties, imposes limitations on optional properties, and can be challenging for developers who are not familiar with TypeScript.

How do I use strictPropertyInitialization in TypeScript?

To use strictPropertyInitialization, set the strictPropertyInitialization flag to true in your TypeScript configuration file. Declare and initialize all properties during the constructor or use default values. Additionally, use the question mark syntax (?) to declare optional properties.

What are the benefits of using strictPropertyInitialization in large projects?

StrictPropertyInitialization helps reduce the likelihood of runtime errors in production, simplifies the debugging process, and improves code quality. In larger projects with multiple developers, strictPropertyInitialization can help enforce consistent code quality standards.

Similar Posts