10 JavaScript Coding Mistakes with solutions
Not declaring variables properly:
Using the wrong comparison operator:
Not using semicolons:
Not understanding scoping:
Using “var” instead of “let” or “const”:
Instead, use “let” or “const”:
Using “==” instead of “===”:
Not using curly braces in if statements:
Using “parseFloat” instead of “parseInt”:
Not handling asynchronous code properly:
Example using a promise:
Example using async/await:
Not properly scoping variables:
Example using let:
Example using var:
Not declaring variables properly:
One of the most common mistakes in JavaScript is forgetting to declare a variable using the “var”, “let”, or “const” keyword. This can lead to unexpected results and make debugging difficult.
Example:
x = 10; // This variable is not declared
console.log(x); // Output: 10
Instead, declare the variable using “var”, “let”, or “const”:
let x = 10;
console.log(x); // Output: 10
Using the wrong comparison operator:
Using the wrong comparison operator can lead to unexpected results, especially when comparing strings and numbers. The “===” operator checks for both value and data type equality, while the “==” operator only checks for value equality.
Example:
console.log(1 == ‘1’); // Output: true
console.log(1 === ‘1’); // Output: false
Not using semicolons:
JavaScript uses semicolons to separate statements. While the interpreter will automatically insert them in most cases, it’s best practice to include them yourself to avoid unexpected results.
Example:
let x = 10
let y = 20
console.log(x + y) // Output: 30
Instead, add semicolons:
let x = 10;
let y = 20;
console.log(x + y); // Output: 30
Not understanding scoping:
Variables declared inside a function have local scope and are not accessible outside of the function. Variables declared outside of a function have global scope and can be accessed from anywhere in the code.
Example:
let x = 10;
function myFunction() {
let x = 5;
console.log(x); // Output: 5
}
myFunction();
console.log(x); // Output: 10
Using “var” instead of “let” or “const”:
“var” declares a variable with function scope, while “let” and “const” declare variables with block scope. Using “var” can lead to unexpected results when trying to access variables outside of their intended scope.
Example:
function myFunction() {
if (true) {
var x = 5;
}
console.log(x); // Output: 5
}
myFunction();
console.log(x); // Output: ReferenceError: x is not defined
Instead, use “let” or “const”:
function myFunction() {
if (true) {
let x = 5;
}
console.log(x); // Output: ReferenceError: x is not defined
}
myFunction();
console.log(x); // Output: ReferenceError: x is not defined
Using “==” instead of “===”:
As mentioned earlier, “==” only checks for value equality, while “===” checks for both value and data type equality. Using “==” can lead to unexpected results.
Example:
console.log(1 == ‘1’); // Output: true
Instead, use “===”:
console.log(1 === ‘1’); // Output: false
Not using curly braces in if statements:
While it’s possible to omit curly braces in if statements when there’s only one statement, it’s best practice to include them to avoid confusion.
Example:
if (x > 10)
console.log(‘x is greater than 10’);
Instead, use curly braces:
if (x > 10) {
console.log(‘x is greater than 10’);
}
Using “parseFloat” instead of “parseInt”:
“parseFloat” is used to convert a string to a floating-point number, while “parseInt” is used to convert a string to an integer. Using the wrong method can lead to unexpected results.
Example:
console.log(parseFloat(‘10.5’)); // Output: 10.5
console.log(parseInt(‘10.5’)); // Output: 10
Not handling asynchronous code properly:
JavaScript is a single-threaded language, but it can handle asynchronous tasks using callbacks, promises, and async/await. Not handling asynchronous code properly can lead to race conditions and other bugs.
Example using a callback:
function myFunction(callback) {
setTimeout(function() {
callback();
}, 1000);
}
myFunction(function() {
console.log(‘Callback executed after 1 second’);
});
Example using a promise:
function myFunction() {
return new Promise(function(resolve, reject) {
setTimeout(function() {
resolve(‘Promise resolved after 1 second’);
}, 1000);
});
}
myFunction().then(function(result) {
console.log(result);
});
Example using async/await:
async function myFunction() {
try {
const result = await new Promise(function(resolve, reject) {
setTimeout(function() {
resolve(‘Async/await resolved after 1 second’);
}, 1000);
});
console.log(result);
} catch (error) {
console.error(error);
}
}
myFunction();
Not properly scoping variables:
When declaring variables, it’s important to declare them within the proper scope, otherwise they may not be accessible where they need to be.
Example:
function myFunction() {
var x = 1; // x is declared in the function scope
if (true) {
var x = 2; // x is now 2, and it’s also in the function scope
}
console.log(x); // Output: 2
}
myFunction();
To properly scope variables, you can use the “let” and “const” keywords introduced in ES6.
Example using let:
function myFunction() {
let x = 1; // x is declared in the function scope
if (true) {
let x = 2; // x is now 2, but it’s only in the block scope
}
console.log(x); // Output: 1
}
myFunction();
Example using var:
function myFunction() {
var x = 1; // x is declared in the function scope
if (true) {
var x = 2; // x is now 2, and it’s also in the function scope
}
console.log(x); // Output: 2
}
myFunction();
In this example, x is declared using the var keyword inside the function myFunction(). When the if block is executed, x is assigned a new value of 2, which changes the value of x outside the if block as well. This is because variables declared with var have function-level scope, meaning they are accessible throughout the entire function, even if they are declared inside a block.
To properly scope variables, you can use the let and const keywords introduced in ES6. Here’s an example:
function myFunction() {
let x = 1; // x is declared in the function scope
if (true) {
let x = 2; // x is now 2, but it’s only in the block scope
}
console.log(x); // Output: 1
}
myFunction();
In this example, x is declared using the let keyword inside the function myFunction(). When the if block is executed, a new variable called x is created with a value of 2, but this variable is only accessible inside the block. The original variable x with a value of 1 is still accessible outside the block and is outputted to the console.