Common JavaScript Mistakes

JavaScript is a flexible and powerful language, but that flexibility can lead to mistakes—especially for beginners. Even experienced developers can fall into certain traps if they’re not careful. Understanding the most common mistakes in JavaScript will help you write cleaner, more reliable code.

In this tutorial, we’ll go through some of the most frequent JavaScript errors and how to avoid them.

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1. Using var Instead of let or const

Using var can lead to confusing behavior due to function scope and hoisting. Modern JavaScript encourages using let for variables that change and const for variables that don’t.

Mistake:

javascriptCopyEditvar count = 10;

Correct:

javascriptCopyEditlet count = 10;     // If the value may change
const MAX = 100;    // If the value stays constant

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2. Forgetting to Declare Variables

If you assign a value to an undeclared variable, JavaScript creates a global variable, which can cause unexpected bugs.

Mistake:

javascriptCopyEditfunction updateScore() {
  score = 100; // Creates a global variable if not declared
}

Correct:

javascriptCopyEditfunction updateScore() {
  let score = 100; // Local scope
}

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3. Confusing == with ===

Using == (loose equality) can lead to unexpected type coercion. Always use === (strict equality) to avoid subtle bugs.

Mistake:

javascriptCopyEditif ("5" == 5) {
  // true, but not recommended
}

Correct:

javascriptCopyEditif ("5" === 5) {
  // false, as it should be
}

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4. Misunderstanding Asynchronous Code

JavaScript is asynchronous by nature, but many developers assume it runs line by line synchronously.

Mistake:

javascriptCopyEditlet result = fetchData();
console.log(result); // Undefined or Promise

Correct (with async/await):

javascriptCopyEditasync function loadData() {
  let result = await fetchData();
  console.log(result);
}

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5. Ignoring Error Handling

Forgetting to handle errors in API calls or JSON parsing can crash your app.

Mistake:

javascriptCopyEditconst data = JSON.parse(userInput); // Might throw an error

Correct:

javascriptCopyEdittry {
  const data = JSON.parse(userInput);
} catch (error) {
  console.error("Invalid JSON:", error.message);
}

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6. Using Magic Numbers or Strings

Hardcoding values without explanation makes code harder to read and maintain.

Mistake:

javascriptCopyEditif (score > 80) {
  console.log("Pass");
}

Correct:

javascriptCopyEditconst PASSING_SCORE = 80;
if (score > PASSING_SCORE) {
  console.log("Pass");
}

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7. Modifying Objects or Arrays Directly

In frameworks like React or functional programming styles, directly mutating state can cause bugs.

Mistake:

javascriptCopyEdituser.age = 30; // Direct mutation

Correct:

javascriptCopyEditconst updatedUser = { ...user, age: 30 }; // Immutable update

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8. Not Using break in Switch Statements

Forgetting break in a switch leads to fall-through bugs where multiple cases run.

Mistake:

javascriptCopyEditswitch (fruit) {
  case "apple":
    console.log("Apple");
  case "banana":
    console.log("Banana");
}

Correct:

javascriptCopyEditswitch (fruit) {
  case "apple":
    console.log("Apple");
    break;
  case "banana":
    console.log("Banana");
    break;
}

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9. Overusing Global Variables

Global variables can easily be overwritten or conflict with other code. Always use local variables where possible.

Mistake:

javascriptCopyEditlet data = []; // Declared globally

Correct:

javascriptCopyEditfunction processData() {
  let data = []; // Local scope
}

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10. Not Testing Code

Relying on “it works in the browser” isn’t enough. Always test with different inputs, edge cases, and tools like console.log() or browser DevTools.

Mistake:

javascriptCopyEdit// Assuming function always works
calculateTotal(100);

Correct:

javascriptCopyEditconsole.log(calculateTotal(100)); // Test output

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Conclusion

JavaScript is a forgiving language, but small mistakes can lead to big problems. By understanding and avoiding these common errors, you’ll write safer, cleaner, and more reliable code.

To recap:

• Use let and const correctly.
• Always declare variables.
• Use strict equality.
• Handle async code with async/await.
• Catch and handle errors.
• Avoid magic values and direct mutations.
• Don’t forget break in switch statements.
• Minimize use of global variables.
• Always test and debug your code.