This article takes you into the world of js data types and data structures

1. What is dynamic typing?

• JavaScript is a weakly typed or dynamic language.
• We do not need to declare the type of the variable in advance, the type will be automatically determined during the program execution.
• This also means that you can use the same variable to store different types of data.

let a; // Initially no type is given a = 11; // a is now of type number a = "二大爷"; // a becomes a string type a = true; // a becomes a Boolean type

It is not recommended to do this in actual application, as it may cause some unpredictable errors.

• For example, your brother suddenly becomes a woman, and is very beautiful ( To sleep or not to sleep? ）
• For example, your best friend suddenly becomes a man, very handsome ( To go or not to go? ）
• For example, the neighbor Mr. Wang becomes your wife... (Mr. Wang himself becomes a cuckold?)
• …
• To sum up, once the data type is determined, do not tamper with it easily ~~
  

2. Data Types

• You may still think that JavaScript only has 6 data types (this is the inherent perception)
• The latest ECMAScript standard defines 8 data types :

2.1 Primitive types (6 primitive types, checked using the typeof operator)

• undefined: typeof instance === "undefined"
• Boolean: typeof instance === "boolean"
• Number: typeof instance === "number"
• String: typeof instance === "string"
• BigInt: typeof instance === "bigint" ( arbitrarily large integer )
• Symbol: typeof instance === "symbol"

2.2 null and Object

• null: typeof instance === "object"
• typeof instance === "object".
• Any special non-data structure type that is an instance of a constructed object, also used as a data structure : (new Object, new Array, new Map, new Set, new WeakMap, new WeakSet, new Date), and almost anything created with the new keyword.

2.3 Key points of typeof operator

• The sole purpose of the typeof operator is to check data types.
• If we want to check the type of any structure derived from Object, using typeof will not work because we will always get "object".
• The proper way to check the type of an object is to use the instanceof keyword (array vs object, etc.), but even this has inaccuracies (there is no 100% ready method at the moment, unfortunately)

3. Original value

3.1 Basic Concepts of Primitive Values

• All types except Object are immutable (the value itself cannot be changed).
• For example, unlike C, strings in JavaScript are immutable ( operations on strings in JavaScript always return a new string, and the original string is not changed ). We call values ​​of these types "primitive values".

3.2 Description of each type

• Boolean type : a logical entity that can have two values: true and false
• Null type : has only one value: null, which means nothing
• Undefined type : A variable that has not been assigned a value will have a default value of undefined
• Number type : There is no further subdivision like Java, both integers and floating point numbers are possible. There are also some signed values: +Infinity, -Infinity, and NaN (Not-a-Number)
• BigInt type : A basic numeric type in JavaScript that can represent integers with arbitrary precision. Large integers can be safely stored and manipulated, even those exceeding the safe integer limit for numbers. A BigInt is created by appending n to the end of an integer or by calling the constructor.

Syntax: BigInt(value);

value: The value of the created object. Can be a string or an integer

BigInt() is not a constructor, so the new operator cannot be used.

• String type : Strings are immutable. This means that once a string is created, it cannot be modified. However, it is possible to create new strings based on operations on original strings.

You can get a substring of a string by selecting individual letters or using String.substr().

To concatenate two strings use the concatenation operator (+) or String.concat().

• Symbols type : unique and unmodifiable, and can also be used as the key value of an Object
• Object type : An object can be viewed as a collection of properties. When an object is defined using object literal syntax, a set of properties is automatically initialized.

The value of a property can be of any type, including objects with complex data structures. Attributes are identified by keys, whose values ​​can be strings or symbols.

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