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Flow: A lightweight but gradually declining choice
Home Web Front-end JS Tutorial JavaScript Type Checking with Flow and TypeScript

JavaScript Type Checking with Flow and TypeScript

Jul 18, 2025 am 12:25 AM

JavaScript requires type checking to cause errors due to dynamic types, and TypeScript and Flow can detect problems in advance. 1.TypeScript is a JS superset, with a large community, a perfect ecosystem, strong compatibility, suitable for new projects or teams to support technology stacks; 2. Flow is lightweight and has little invasiveness, suitable for gradual migration of old projects, but has low community activity. Selection basis: Use TypeScript for new projects, and use Flow for old projects. At the same time, consider framework support such as Angular to adapt to TS is better.

JavaScript Type Checking with Flow and TypeScript

If you often encounter variable types when writing JavaScript code, errors in passing parameters, or undefined problems when running the time, you need a more reliable way to manage the types. Flow and TypeScript are both mainstream JavaScript type checking tools nowadays, and they can help you find problems in advance. However, which one to choose and how to use it depends on your project needs and team habits.

JavaScript Type Checking with Flow and TypeScript

Why JavaScript requires type checking

JavaScript is dynamic type, although flexible, but it is easy to bury. For example, if you write a function and you originally expected to receive a string, but if someone passes a number in, the program may crash. This kind of error is particularly prone to development of large applications.

Type checking tools (such as Flow and TypeScript) help you check "whether the parameters meet expectations" when you write code, and report errors in advance instead of waiting until runtime to find problems. Moreover, type annotation itself is a kind of document that can help others understand your code structure faster.

JavaScript Type Checking with Flow and TypeScript

TypeScript: A comprehensive and popular type system

TypeScript is an open source language launched by Microsoft. It is actually a superset of JavaScript, which means that all legitimate JS code is legitimate TS code.

advantage:

JavaScript Type Checking with Flow and TypeScript
  • The community is large and the ecology is perfect. Many frameworks (such as React and Vue) are natively supported
  • Supports compilation into different versions of JS, which is convenient for compatibility with old environments
  • IDE support is good, VSCode is almost seamlessly integrated

How to use:

  1. Install typescript package
  2. Create tsconfig.json configuration file
  3. Rename the .js file to .ts
  4. Add type annotations, for example:
 function greet(name: string): string {
  return 'Hello, ' name;
}

Flow: A lightweight but gradually declining choice

Flow is a type checker launched by Facebook. It does not require you to change the file suffix or build too much. Just add a sentence // @flow to the JS file to start using type annotations.

advantage:

  • Less intrusive to existing JS projects, suitable for progressive migration
  • Supports some advanced type features, such as precise object types, etc.

shortcoming:

  • Community activity is not as good as TypeScript, and the update frequency is reduced
  • The toolchain and third-party library support is relatively less

Simple example:

 // @flow
function square(n: number): number {
  return n * n;
}

Flow does not force check all files by default, only those with @flow annotation will be processed, which is more "gentle" than TypeScript.


How to choose Flow or TypeScript

If you are:

  • Start a new project, recommend TypeScript, which has obvious community and ecological advantages
  • When maintaining an old JS project, if you want to gradually introduce type checks, you can first use Flow to test the water temperature
  • Someone in the team is already familiar with one of them, so follow the existing technology stack

You can also see if the framework you are using is better for a tool. For example, Angular strongly recommends TypeScript. Although React supports both, the official documentation is more inclined to TS.


Basically that's it. Type checking is not a silver bullet, but it can help you reduce a lot of low-level errors and also make the code easier to maintain. Whether it is Flow or TypeScript, choose one to get started, and you will find that you feel much more at ease when writing JS.

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