JavaScript Weird Parts and Quirks

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JavaScript Weird Parts and Quirks

1. Hoisting of Variables and Functions

Hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their containing scope (global or function scope) during the compilation phase, before code execution.

How Hoisting Works

1. Variables declared with var are hoisted but not initialized.
2. let and const are also hoisted but remain in a Temporal Dead Zone (TDZ) until the line where they are declared.
3. Function Declarations are fully hoisted, which allows them to be called before their definition.

Examples of Hoisting

1. Hoisting with var

console.log(x); // Output: undefined (hoisted but not initialized)

var x = 10;

console.log(x); // Output: 10

Explanation:

• The var x is hoisted to the top, but only the declaration is hoisted, not its initialization.
• So, the first console.log(x) outputs undefined.

2. Hoisting with let and const

console.log(y); // ReferenceError: Cannot access 'y' before initialization

let y = 20;

Explanation:

• let and const are hoisted but exist in the Temporal Dead Zone.
• Accessing y before the declaration throws a ReferenceError.

3. Hoisting of Functions

greet(); // Output: Hello!

function greet() {

console.log('Hello!');

}

Explanation:

• Since function declarations are fully hoisted, the function can be called before its definition.

2. Type Coercion and Loose Equality (== vs ===)

Type coercion is the automatic conversion of values from one data type to another.

• == (loose equality): Converts values to the same type before comparing.
• === (strict equality): No type conversion; values and types must both be the same.

Examples of Type Coercion

1. Loose Equality (==)

console.log(5 == '5'); // true (because '5' is converted to a number)

console.log(false == 0); // true (false is coerced to 0)

console.log('' == 0); // true (empty string is coerced to 0)

2. Strict Equality (===)

console.log(5 === '5'); // false (no type conversion)

console.log(false === 0); // false (different types)

console.log('' === 0); // false (empty string and 0 are not the same)

3. NaN and Its Unexpected Behavior

NaN (Not-a-Number) is a special numeric value that represents invalid mathematical operations.

Why is NaN Weird?

• NaN is not equal to NaN.
• typeof NaN returns "number".

Examples of NaN Quirks

console.log(NaN === NaN); // false

console.log(typeof NaN); // 'number'

console.log(Number('abc')); // NaN

How to Check for NaN

Use Number.isNaN() to check for NaN (better than isNaN()).

console.log(Number.isNaN(NaN)); // true

console.log(Number.isNaN('abc')); // false

4. Falsy and Truthy Values in JavaScript

Truthy and falsy values affect conditionals (if, while, etc.).

Falsy Values

• false
• 0
• '' (empty string)
• null
• undefined
• NaN

Truthy Values

• All values except falsy values.

Example of Falsy Values

if (0) {

console.log('This will not run');

}

if ('') {

console.log('This will not run');

}

Example of Truthy Values

if ([]) {

console.log('This will run');

}

if ('hello') {

console.log('This will run');

}

5. Floating-Point Arithmetic Errors

JavaScript uses binary floating-point arithmetic (IEEE 754), which causes issues with precision.

Examples of Floating-Point Issues

console.log(0.1 + 0.2); // 0.30000000000000004

console.log(0.1 + 0.7 === 0.8); // true

Solution

Use toFixed() or multiplication workaround.

console.log((0.1 10 + 0.2 10) / 10); // 0.3

6. The Arguments Object and Its Quirks

The arguments object is an array-like object that holds all arguments passed to a function.

Example of Arguments Object

function sum() {

console.log(arguments);

}

sum(1, 2, 3); // [1, 2, 3]

Quirks of Arguments

1. Not available in arrow functions.
2. Not a real array — no .map() or .filter().
3. Use Array.from(arguments) to convert it to a real array.

7. How Arrays Behave with delete and Empty Slots

What Happens When You Use delete on Arrays?

const fruits = ['Apple', 'Banana', 'Cherry'];

delete fruits[1];

console.log(fruits); // ['Apple', <empty slot>, 'Cherry']

• delete removes the element but leaves an empty slot.
• Instead, use splice().

Solution

const fruits = ['Apple', 'Banana', 'Cherry'];

fruits.splice(1, 1);

console.log(fruits); // ['Apple', 'Cherry']

8. Differences Between var, let, and const

var

let

const

Function Scope

Block Scope

Block Scope

Can be redeclared

Cannot be redeclared

Cannot be redeclared

Can be updated

Can be updated

Cannot be updated

Hoisted

Hoisted (TDZ)

Hoisted (TDZ)

Examples of var, let, and const

var x = 10;

let y = 20;

const z = 30;

9. The Misleading typeof null

Why Does typeof null Return "object"?

This is a bug in JavaScript. Originally, JavaScript stored internal data types as type tags, and null was mistakenly assigned as object.

Example

console.log(typeof null); // 'object'

How to Check for null Correctly

if (value === null) {

console.log('Value is null');

}

10. Automatic Semicolon Insertion (ASI)

JavaScript automatically inserts semicolons at the end of statements, but this can lead to subtle bugs.

Problematic Case

return

{

name: 'Alice'

}

Output: undefined (because JavaScript inserts a semicolon after return)

Solution

return {

name: 'Alice'

};

Summary of Key Concepts

Concept

Description

Hoisting

Variables/functions moved to the top

Type Coercion

Automatic conversion of data types

NaN

Not-a-Number, but of type "number"

Falsy Values

Values that convert to false in Boolean

Floating-Point Errors

Inaccurate decimal calculations

delete in Arrays

Leaves empty slots in arrays

typeof null

Returns "object" due to legacy issues

JavaScript Weird Parts and Quirks 

1. Global Object Quirk (this in Global Context)

What is the Global Object?

The global object refers to the "top-level" object in a JavaScript environment. Its name differs depending on the context:

• In browsers, the global object is window.
• In Node.js, the global object is global.

Weird Behavior of this in the Global Context

console.log(this); // In browser, it logs the 'window' object.

console.log(this === window); // true (only in browser)

Quirky Behavior

• In the global scope, this points to the global object.
• In strict mode, this is undefined in the global scope.

Example in Strict Mode

'use strict';

console.log(this); // undefined

This prevents accidental use of the global object, a common source of bugs.

2. with Statement (Why You Should Avoid It)

The with statement extends the scope chain for a specific object, making it a bad practice.

How it Works

const person = { name: 'Alice', age: 25 };

with (person) {

console.log(name); // Alice

console.log(age); // 25

}

Why It's a Quirk

• Ambiguity: The variable name could be from the person object or from the global scope.
• Performance Issues: It changes the scope chain, slowing down execution.

Best Practice

Avoid using with as it is discouraged and might be removed in the future.

3. Implicit Type Conversion (More Weird Coercion Cases)

Implicit type coercion happens when JavaScript converts one type to another during operations.

Strange Coercion Examples

console.log(true + false); // 1 (true = 1, false = 0)

console.log('5' - 3); // 2 ('5' becomes 5)

console.log('5' + 3); // '53' (concatenation, not arithmetic)

console.log([1, 2] + [3, 4]); // '1,23,4' (arrays become strings)

Coercion with Objects

const obj = { toString() { return 'Hello'; } };

console.log(obj + ' World'); // 'Hello World'

Explanation: The object is converted to a string using its toString() method.

4. Strange Behavior of Empty Arrays and Empty Objects

Array Quirks

console.log([] + []); // '' (two empty arrays are converted to empty strings)

console.log([] + {}); // '[object Object]' (empty array becomes empty string, {} becomes "[object Object]")

console.log({} + []); // 0 (parsed as an empty code block, and the addition of [] as a number gives 0)

Object Quirks

console.log({} + {}); // [object Object][object Object]

Explanation:

• If you add two objects with +, it coerces them to strings using Object.prototype.toString(), which returns "[object Object]".

5. Weird Loops (for...in vs. for...of)

for...in (Iterates Over Keys)

const array = ['a', 'b', 'c'];

for (const key in array) {

console.log(key); // Logs 0, 1, 2 (the indices)

}

Explanation:

• for...in iterates over keys (property names), even on arrays.

for...of (Iterates Over Values)

for (const value of array) {

console.log(value); // Logs 'a', 'b', 'c'

}

Explanation:

• for...of iterates over values in iterable objects (like arrays, strings, etc.).

6. The Quirk of Floating-Point Modulus

Due to floating-point precision errors, modulus operations don't always return expected results.

Example

console.log(0.3 % 0.1); // 0.09999999999999995 (not 0)

Explanation:

• JavaScript’s floating-point system is imprecise.
• To avoid this, use integer arithmetic.

7. Function Length Property

The length property of a function tells you how many parameters the function expects.

Example

function foo(a, b, c) {}

console.log(foo.length); // 3 (number of expected parameters)

8. The Quirk of Adding Properties to Primitives

Example

const str = 'Hello';

str.language = 'English';

console.log(str.language); // undefined

Explanation:

• Primitive values (like strings) are not objects.
• Temporary wrapper objects are created for methods like .length and .toUpperCase(), but they disappear immediately.

9. The Quirk of isNaN()

Problem with isNaN()

console.log(isNaN('abc')); // true (because 'abc' is coerced to NaN)

console.log(isNaN(NaN)); // true

Explanation:

• isNaN() converts the argument to a number and checks if it’s NaN.
• Instead, use Number.isNaN() for a strict check.

Solution

console.log(Number.isNaN('abc')); // false

console.log(Number.isNaN(NaN)); // true

10. Quirky Array Sorting

Problem with Default Array Sort

const nums = [1, 10, 2, 21];

nums.sort();

console.log(nums); // [1, 10, 2, 21] (sorted as strings)

Solution

nums.sort((a, b) => a - b); // [1, 2, 10, 21]

Explanation:

• The default sort method converts items to strings and compares character codes.

11. The Quirk of Empty Return Statements

Problem

function doSomething() {

return

{ message: 'Hello' };

}

console.log(doSomething()); // undefined

Explanation:

• A semicolon is automatically inserted after return, causing the object to be unreachable.

Solution

function doSomething() {

return {

message: 'Hello'

};

}

12. Quirk of Infinity

Quirks

• Positive Infinity and Negative Infinity exist.
• Division by zero gives Infinity.
• You can check if a number is finite using isFinite().

Example

console.log(1 / 0); // Infinity

console.log(-1 / 0); // -Infinity

13. Weird Logical Operators

Logical operators have some unexpected behavior.

Examples

console.log(true || 'Hello'); // true

console.log(false || 'Hello'); // 'Hello'

console.log(false && 'Hello'); // false

console.log(true && 'Hello'); // 'Hello'

Summary of Key Concepts

Concept

Description

Global Object

this points to the global object

Implicit Coercion

Auto-conversion of types

Floating-Point Errors

Imprecision in arithmetic

Weird Array Sort

Array.sort() sorts by strings

Logical Operators

Unexpected short-circuit behavior

Hoisting

Variables/functions move to the top

Falsy Values

Values that convert to false in Boolean

Automatic Semicolon

Can cause unintended return issues

JavaScript Weird Parts and Quirks (Ultimate Guide)

1. Hoisting of Variables and Functions (Revisited)

Hoisting quirks often confuse developers because of how declarations are lifted to the top of their scope.

Unusual Hoisting Behavior with var

function hoistQuirk() {

console.log(a); // undefined

var a = 10;

}

hoistQuirk();

Explanation:

• The declaration var a is hoisted to the top, but its initialization (a = 10) is not.
• Hence, console.log(a) returns undefined.

Unusual Hoisting with Functions

function example() {

console.log(foo()); // Works because foo is hoisted

function foo() {

return 'Hello, World!';

}

}

example();

Explanation:

• Function declarations are fully hoisted, allowing them to be called before they are defined.

2. Type Coercion Oddities

Type coercion leads to some very unusual and counter-intuitive results.

Bizarre Coercion Examples

console.log([] + []); // '' (empty arrays are converted to empty strings)

console.log({} + []); // '[object Object]' ({} is converted to string)

console.log([] + {}); // '[object Object]' (empty array becomes '', {} is converted to string)

console.log(true + true); // 2 (true is converted to 1)

console.log('5' - 1); // 4 ('5' is converted to 5)

Solutions to Avoid Coercion

1. Use strict equality (===).
2. Explicitly convert data types using Number(), String(), and Boolean().

3. NaN (Not-a-Number) Paradoxes

Weird NaN Examples

console.log(NaN === NaN); // false (NaN is not equal to itself)

console.log(typeof NaN); // 'number' (even though it's "Not-a-Number")

console.log(0 / 0); // NaN

How to Handle NaN

1. Use Number.isNaN() for strict checks.
2. Avoid coercion-based checks like isNaN().

4. Falsy and Truthy Values (Surprising Ones)

Surprising Falsy Values

• false
• 0 (zero)
• '' (empty string)
• null
• undefined
• NaN

Surprising Truthy Values

• [] (empty array)
• {} (empty object)
• Functions (all functions are truthy)
• Non-empty strings ('0', 'false', etc.)

Quirky Falsy Value Example

if ('false') {

console.log('This will run'); // True (because 'false' is a non-empty string)

}

5. Floating-Point Precision Issues (Revisited)

More Examples of Floating-Point Errors

console.log(0.1 + 0.2); // 0.30000000000000004 (not 0.3)

console.log(0.3 - 0.1); // 0.19999999999999998 (not 0.2)

console.log(1.005.toFixed(2)); // '1.00' (not '1.01')

Solution

const preciseSum = (a, b) => Math.round((a + b) * 100) / 100;

console.log(preciseSum(0.1, 0.2)); // 0.3

6. Strange Behavior of delete on Arrays

Weird delete Examples

const arr = [1, 2, 3];

delete arr[1];

console.log(arr); // [1, <empty>, 3] (not [1, 3])

console.log(arr.length); // 3 (length does not change)

Solution

Use splice() instead of delete.

const arr = [1, 2, 3];

arr.splice(1, 1);

console.log(arr); // [1, 3]

7. The Misleading typeof null

Why typeof null Returns "object"

console.log(typeof null); // 'object'

Explanation:

• This is a legacy bug in JavaScript.
• null was supposed to be a unique "null" type, but it mistakenly got labeled as an "object".

Solution

if (value === null) {

console.log('Value is null');

}

8. Quirky Array Sorting

Strange Sorting Behavior

const numbers = [1, 10, 2, 21];

numbers.sort();

console.log(numbers); // [1, 10, 2, 21] (not sorted as expected)

Solution

numbers.sort((a, b) => a - b);

console.log(numbers); // [1, 2, 10, 21]

9. Automatic Semicolon Insertion (ASI) Issues

Unexpected Behavior

function doSomething() {

return

{

message: 'Hello'

};

}

console.log(doSomething()); // undefined (because ASI inserts a semicolon after return)

Solution

function doSomething() {

return {

message: 'Hello'

};

}

10. Function Quirks (Default Parameters and Rest/Spread)

Default Parameter Quirks

function greet(name = 'Guest') {

console.log(`Hello, ${name}`);

}

greet(); // Hello, Guest

greet(undefined); // Hello, Guest (because undefined triggers the default)

greet(null); // Hello, null (null is not undefined)

Rest Parameters

function sum(...nums) {

return nums.reduce((total, num) => total + num, 0);

}

console.log(sum(1, 2, 3, 4)); // 10

11. Quirk of Infinity

Examples of Infinity

console.log(1 / 0); // Infinity

console.log(-1 / 0); // -Infinity

Explanation:

• Division by zero returns Infinity.
• Infinity behaves like a number in many contexts.

Check for Infinity

console.log(isFinite(1 / 0)); // false

console.log(isFinite(10 / 2)); // true

12. Quirky Object Property Keys

Property Keys as Strings

const obj = { 1: 'one', true: 'boolean', null: 'null' };

console.log(obj['1']); // 'one'

console.log(obj['true']); // 'boolean'

console.log(obj['null']); // 'null'

Why This Happens

• Object property keys are always strings.
• If you use a number, it’s converted to a string ('1').

13. The Quirk of this in Arrow Functions

Quirky Example

const person = {

name: 'Alice',

greet: () => {

console.log(`Hello, my name is ${this.name}`);

}

};

person.greet(); // Hello, my name is undefined

Explanation:

• Arrow functions do not have their own this.
• Arrow functions inherit this from the lexical environment.

Summary of Key Concepts

Concept

Description

Type Coercion

Odd conversions ('5' + 1 = '51')

NaN

Not-a-Number is of type "number"

delete on Arrays

Leaves "holes" in arrays

typeof null

Returns "object" due to legacy issues

ASI

Automatic semicolon insertion

Infinity

Division by zero returns Infinity