null. nan. undefined

# Web Site Writing Basics

# javascript

# null_ nan _ undefined

The null variable in JavaScript is a value that represents no intentional value or that the variable does not point to any object.

Important characteristics of null

Data Type (Type): Although null means' no value ', when using the typeof operator with null, it results as' object ', a mistake that has occurred in the history of JavaScript and remains.

JavaScript

typeof null; / / The result is' object'

The Falsy: null value is considered a falsy value, which means that when converted to a boolean (boolean) value, the result is false.

Differences from undefinite:

The null is configured by the developer to indicate that the variable has no value.

Undefined means a variable has been declared but has not yet been configured or access to a property that does not exist.

Example of use

You may use null to:

Default value defined: requires the variable to start with a known 'empty' value.

JavaScript

Let selectedUser = null; / / The selected user does not have

Clear Reference Value: Remove references from objects to help with memory management.

JavaScript

Let data aObject = {name: "Alice"};

/ /....Use data aObject...

Data aObject = null; / / Clears references when this object is no longer needed.

Comparison

Comparison. Results. Notes.

Null = = undefined true Loose comparison (loose equality) assumes that the two are equal.

Null = = = undefined false A strict comparison (strict equality) assumes that the two have unequal kinds and values.

Null = = = null true -

null = = 0 false null is not equal to 0.

Null > 0 false -

Null > = 0 true When compared mathematically, null is often converted to 0.

Undefined in JavaScript is a value that represents a missing value. It occurs when a variable is declared but has not been defined by default, or when trying to access the property of an object (object) or a member of an array (array) that does not exist.

Important characteristics of undefined

Data Type (Type): The data type of undefined is' undefined '.

JavaScript

typeof undefined; / / The result is' undefined'

Falsy value: undefined is considered a falsy value, which means that when converted to a boolean (boolean) value, the result is false.

Differences from null:

Undefined is automatically the default value when the variable is missing a value (unintended).

Null is a value that the developer intentionally defined to indicate that the variable does not have a valid value or does not point to any object.

An example that causes undefined

Variables declared but without default:

JavaScript

Let name;

console.log (name); / / result: undefined

Access to properties not available in the object:

JavaScript

const user = {id: 101};

console.log (user.email); / / Result: undefined

The non-return function is clear:

JavaScript

greet (name) function {

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

/ / This function does not use the return command.

}

Const result = greet ('Alice');

console.log (result); / / result: undefined

Comparison

Comparison. Results. Notes.

Undefined = = null true Loose comparison (loose equality) assumes that the two are equal.

Undefined = = = null false A strict comparison (strict equality) assumes that the two have unequal kinds and values.

undefined = = = undefined true

NaN in JavaScript, short for "Not a Number" (not a number), is a special value in the JavaScript language that represents the result of a mathematical operation that cannot provide a valid numerical value.

Key characteristics of NaN

Data type (Type): Although it means "not a number," the data type of NaN is' number '.

JavaScript

typeof NaN; / / The result is' number'

Falsy value: NaN is considered a falsy value.

Cause of NaN

NaN usually occurs when you try to perform mathematical operations that do not make sense or cannot. Such as:

Dividing zero by zero:

JavaScript

0 / 0; / / Result: NaN

Mathematical operations on non-numeric values:

JavaScript

'Hello '* 5; / / Result: NaN

10 - 'World'; / / Result: NaN

Trying to convert a non-numerical value into a number:

JavaScript

ParseInt ('apple'); / / Result: NaN

Number ('oops'); / / Result: NaN

Comparison and investigation of NaN

The most important thing to remember about NaN is:

NaN is not equal to itself: this is a special feature of it that is different from other values in JavaScript.

JavaScript

NaN = = NaN; / / Result: false

NaN = = = NaN; / / Result: false

So to determine which values are NaN, you can not use the comparative operator (= = or = = =), but the specific function:

The isNaN () function: is a recommended way to determine if a value is NaN.

JavaScript

isNaN (10); / / Result: false (10 as a number)

isNaN (NaN); / / Result: true

isNaN ('Hello'); / / Result: true (because 'Hello' was tried to convert to a number and then got NaN)

The Number.isNaN () function: It is a more rigorous and better way to determine if a value is NaN without trying to convert a non-numeric value into a number (as isNaN () does).

JavaScript

Number.isNaN (NaN); / / Result: true

Number.isNaN ('Hello'); / / Result: false (because 'Hello' is not a direct NaN value.