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5 Popular JavaScript Mistakes and How to Avoid Them

JavaScript is a powerful and flexible programming language, but it can be prone to mistakes, especially if you don’t follow best practices and ignore its peculiarities. In this article, let’s look at 5 popular mistakes developers often make when working with JavaScript and suggest ways to avoid them.

  1. using == instead of === for comparisons

One of the common mistakes is using the == comparison operator, which performs type conversion. This can lead to unexpected results.

For example:

0 == false; // true
null == undefined; // true
'5' == 5; // true

Using === (strict comparison) will help avoid these problems because it does not perform type conversion:

0 === false; // false
null === undefined; // false
'5' === 5; // false

How to avoid: Always use === for comparisons. This will ensure more predictable behavior of your code.

  1. Variable scope mismatch

JavaScript uses different scopes for variables depending on how they are declared. Errors in scope handling can lead to unexpected results, especially when using variables with the same names.

For example:

function test() {
var x = 10;
}
console.log(x); // Uncaught ReferenceError: x is not defined

If you use let or const in blocks, variables remain available only within those blocks.

if (true) {
let y = 20;
}
console.log(y); // Uncaught ReferenceError: y is not defined

How to avoid: Use let and const instead of var to define variables, and always watch the scope in which they are used.

  1. asynchronous programming errors (callback hell)

Asynchronous operations in JavaScript (for example, working with setTimeout, fetch or promises) can often lead to the so-called “callback hell” – a situation where complex code leads to deeply nested callbacks.

For example:

getData(function(response) {
process(response, function(result) {
save(result, function(saved) {
console.log(saved);
});
});
});

Such code becomes hard to read and difficult to debug.

How to avoid: Use promises or async/await for cleaner and more usable asynchronous code:

async function fetchData() {
const response = await fetchData();
const result = await process(response);
const saved = await save(result);
console.log(saved);
}
  1. Unclosed curly braces or incorrect use of quotation marks

JavaScript is very sensitive to syntax errors. Sometimes forgotten curly braces or incorrect use of quotation marks can lead to hard-to-see bugs.

For example:

if (x > 5)
console.log('X is greater than 5');
console.log('This line will always be executed!');

This code will always execute the second line, even if the condition is not true.

How to avoid: Always follow the correct block structure and use curly braces even for single line statements.

  1. Changing values of variables passed to a function by reference

In JavaScript, data types come in primitive (e.g., strings, numbers) and object (arrays, objects) types. When you pass a primitive to a function, a copy of the value is created, and it does not affect the original variable. However, objects are passed by reference, which can cause the original object to change if not expected.

Example:

function modifyArray(arr) {
arr.push(4);
}
let numbers = [1, 2, 3];
modifyArray(numbers);
console.log(numbers); // [1, 2, 3, 4]

How to avoid: If you don’t want the object or array to change, create a copy of it before passing it to the function:

function modifyArray(arr) {
let newArray = […arr];
newArray.push(4);
return newArray;
}

Conclusion:

Avoiding these errors will help you follow best practices, use modern JavaScript features (such as let, const, async/await), and pay close attention to syntax and code structure. This will make your code more stable, understandable, and easy to debug.

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