2. Tutorials
  3. Deprecate a Method in JavaScript and TypeScript

Deprecate a Method in JavaScript and TypeScript

When writing code, you eventually want or need to change existing implementation. Changing existing code can have side effects for the application and you may need to refactor parts of the project which you didn’t plan to touch.

Yet, it’s necessary to move forward and sunset old versions of your code that you don’t want to maintain anymore. You can do this by deprecating features in JSDoc.

This tutorial shows you how to deprecate a method in JavaScript. This deprecation process also works in TypeScript, too.

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Deprecate an Individual Function

Your application may export individual utility functions. For example, you may have a readFileContentsAsString that reads a file from the local hard drive and returns the file’s content as a string value.

Let’s say you want to change the name of the function to contents. A good approach is to keep the old readFileContentsAsString around in your code and mark it as deprecated.

JavaScript supports JSDoc annotation to mark a property or method as deprecated. Use the @deprecated directive in a block comment above your method:

/**
 * @deprecated This method is deprecated in favor of the {@link contents} method. We’re removing this method in version 2.0.
 */
export function readFileContentsAsString (filePath) {  
  return contents(filePath, { encoding: 'utf8' })
}

export function contents (filePath, options) {  
  // the actual implementation …
}

You may also use the JSDoc @link annotation in combination with a deprecation. The @link annotation creates a clickable reference in the editor to the linked, new implementation.

Here’s a screenshot of the sample code in Visual Studio Code. The important part is the strikethrough of the readFileContentsAsString method highlighting the deprecation of that method:

Deprecated function in VS Code: highlighted with a strikethrough

You can hover the deprecated function in Visual Studio Code to get a popover with more details for that method. Your deprecation message is available in the popover section at the bottom. It contains the clickable link to the contents methods:

Deprecation details in a VS Code popover

Add a Log Line to Surface the Deprecation

You can surface deprecations with the help of log lines. Depending on your context, you may or may not add a log line with specifics. For example, if you’re deprecating a method in a library that’s used in the browser context, it’s possibly better to not log deprecation messages to avoid spamming the developer console.

We like having messages that point out the usage of deprecated messages. It helps us to notice possible breaking changes in the future because the package developer will remove something we’re currently using:

/**
 * @deprecated This method is deprecated in favor of the {@link contents} method. We’re removing this method in version 2.0.
 */
export function readFileContentsAsString (filePath) {  
  console.log('This [readFileContentsAsString] method is deprecated and will be removed in version 2.0. Please use the [contents] method instead. ')

  return contents(filePath, { encoding: 'utf8' })
}

export function contents (filePath, options) {  
  // the actual implementation …
}

Deprecate a Class Method

You can also deprecate individual methods in a JavaScript class. It works the same way: use the @deprecate JSDoc directive and provide a deprecation message with more details:

export class FileSystem {  
  /**
   * Returns the contents of the file located at `filePath` as a string.
   *
   * @param {String} filePath
   * @returns {String}
   *
   * @deprecated use the {@link contents} method
   */
  readFileContentsAsString (filePath) {
    return this.contents(filePath, { encoding: 'utf8' })
  }

  /**
   * Returns the contents of the file located at `filePath`.
   */
  contents (filePath, options) {
    // the actual implementation …
  }
}

We’re putting the @deprecate annotation at the end of a JSDoc block, as the last element. Putting deprecations last is a team consent, in case a coworker needs to find more details for the deprecation, they know where to find them.

Here’s a screenshot from Visual Studio Code using the sample code with the deprecated readFileContentsAsString method. Visual Studio Code highlights the call to a deprecated method with a strikethrough (line 25):

VS Code highlights deprecated methods with a strikethrough

You may hover the deprecated method with the mouse to show a popover with more details. The last section in the popover contains your deprecation message with the link to the contents method. Clicking the link navigates your cursor to the linked method within Visual Studio Code:

VS Code shows the deprecation message with linked methods in a popover

That’s it!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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