tslint / codelyzer / ng lint error: "for (... in ...) statements must be filtered with an if statement"

📝 Blog Post: Quick Fix for "for (... in ...) statements must be filtered with an if statement" Error in TSLint

Are you seeing the dreaded TSLint error message: "for (... in ...) statements must be filtered with an if statement"? Don't worry, you're not alone! This error often pops up when using tools like TSLint, Codelyzer, or ng lint in your Angular projects. But fear not, because in this blog post, we're going to address this common issue and provide you with easy solutions to fix it.

🔎 Understanding the Problem

Let's break down the error message and understand what it's trying to tell us. The error originates from the following code snippet:

for (const field in this.formErrors) {
  // clear previous error message (if any)
  this.formErrors[field] = '';
  const control = form.get(field);

  if (control && control.dirty && !control.valid) {
    const messages = this.validationMessages[field];
    for (const key in control.errors) {
      this.formErrors[field] += messages[key] + ' ';
    }
  }
}

This code is responsible for validating form fields and updating error messages. The issue, however, lies in the usage of the "for (... in ...)" statements without filtering them with an if statement. This can lead to unintended behavior and potential bugs in our code.

🛠️ Quick and Easy Solution

To fix this lint error, we need to add an if statement to filter the "for (... in ...)" iterations. Here's how to modify the code:

for (const field in this.formErrors) {
  // clear previous error message (if any)
  this.formErrors[field] = '';
  const control = form.get(field);

  if (control && control.dirty && !control.valid) {
    const messages = this.validationMessages[field];
    for (const key in control.errors) {
      if (control.errors.hasOwnProperty(key)) { // Add this if statement
        this.formErrors[field] += messages[key] + ' ';
      }
    }
  }
}

By adding the "if (control.errors.hasOwnProperty(key))" check, we ensure that only the valid properties are looped over, preventing any unintended side effects.

💡 Further Explanation

The error occurs because the "for (... in ...)" loop iterates over all the properties of an object, even if they are not directly related to the validation error. Therefore, we need to filter out irrelevant properties while working with form validation or any similar scenario.

In our fixed code, the "if (control.errors.hasOwnProperty(key))" line ensures that the iteration is limited to the actual validation errors present in the control's "errors" object.

🔍 Learn More

If you want to dig deeper into the concepts of form validation in Angular, feel free to check out the official Angular documentation's form validation section here.

📣 Boost Your Development Workflow

Now that you're equipped with the knowledge to fix the "for (... in ...) statements must be filtered with an if statement" error, don't let it slow you down. Make sure to incorporate linting tools like TSLint, Codelyzer, or ng lint into your development workflow. These tools help maintain code quality and catch potential issues early on, resulting in smoother development and better software.

Remember, happy developers write better code! 🚀

🙌 Engage with Us

Have you encountered this lint error before? How did you solve it? What other coding hurdles have you overcome recently? Share your experiences, tips, and tricks in the comments section below. Let's grow together as a community of developers!

📤 Share the Love

If you found this blog post helpful, don't keep it to yourself! Share it with your fellow developers, teammates, or on your favorite social media platform. Let's spread the knowledge and help others conquer this linting error too.

Until next time, happy coding! ✨👩‍💻👨‍💻✨