What are the two data types of JavaScript?

Javascript data types are divided into two types: 1. Value types (basic types), including String, Number, Boolean, Null, Undefined and Symbol types; 2. Reference data types, including Object, Array and Function type.

The operating environment of this tutorial: Windows 7 system, ECMAScript version 6, Dell G3 computer.

javascript data types are divided into:

• Value types (basic types): String, Number, Boolean ( Boolean), Null, Undefined, Symbol.

• Reference data types: Object, Array, Function.

Detailed explanation of basic data types:

1. Number type

①The number type includes integers and floating-point numbers (the floating-point number value must contain a decimal point and at least one digit after the decimal point).

Floating point numbers will be automatically converted to integers, as follows:

let num = 1.00
console.log(num) // 输出：1，自动转换为了整数

② The highest precision of floating point numbers is 17 digits, see the following example:

let a = 0.1
let b = 0.2
console.log(a + b) // 输出：0.30000000000000004

The above example, the result It’s not 0.3. As for other languages, this situation will also occur (there will be errors in floating-point calculations, and similar situations will occur in other languages.)

③ NaN: Not a number Type, characteristics: Any operation involving NaN will return NaN, and NaN is not equal to itself. As follows:

let name = 'lisi'
console.log(name / 10) // 输出：NaN
console.log(NaN === NaN) // 输出：false

④ The isNaN() function is used to determine whether it is a non-numeric type. If the parameter passed in is a non-numeric type, then return true, otherwise return false;

⑤ The isNaN() function passes in a parameter. The function will first convert the parameter into a numerical value. If the parameter type is an object type, it will first call the valueOf() method of the object, and then determine whether the value returned by the method can be converted into a numerical type. If No, call the toString() method of the object again, and then determine the return value;

⑦ Numeric conversion:

Number(), transformation function, can be used for any data type

parseInt(), converts the value to an integer type, it is often used

parseFloat(), converts the value to a floating point type

[Recommended learning: javascript advanced tutorial】

2. String type

① The single and double quotes in the string type have the same effect;

② The string has the length attribute. You can get the length of the string:

let str = 'Hello world'
console.log(str.length) // 输出：11

③ The value of the string is immutable. To change the value of a string, you must first destroy the original string, and then use another string area containing the new value. Fill in the string;

④ String conversion:

String(), transformation function, suitable for any data type (null, undefined is converted to null and undefined)

## The #toString() method can convert a logical value into a string and return the result (null, undefined does not have toString() method)

let ab = 'zhangsan'
let bc = null
let cd = undefined
console.log(ab.toString())  // 输出：zhangsan
// console.log(bc.toString())  // error 报错
// console.log(cd.toString())  // error 报错
console.log(String(ab))  // 输出：zhangsan
console.log(String(bc))  // 输出：null
console.log(String(cd))  // 输出：undefined

3, boolean type

① This type has only two values, true and false;

② Boolean(), transformation function, converts a value into a Boolean type

4, null type

① The null type is regarded as a null object pointer and has only one value, the null value. Therefore, when using the typeof operator to detect the value of the null type, the result is the object type;

② If you define a variable, but want to use the object as an object in the future, it is best to initialize the object to null value

5, undefined type

① There is only one value, which is undefined. If a variable is declared but the variable is not initialized, then the value of the variable is undefined:

let name = 'zhangsan'
let age
console.log(name) // 输出：zhangsan
console.log(age) // undefined，因为声明了age，但是没有给初始化值

② When calling the function, the parameters that should be provided are not provided. The parameter is equal to undefined;

③ The object has no assigned attribute, and the value of the attribute is undefined;

④ The function has no return value and returns undefined by default.

6. Symbol

Symbol is a new primitive data type introduced in ES6 to represent unique values. The Symbol type is unique and cannot be modified

var s = Symbol()

The new command cannot be used before the Symbol function, otherwise an error will be reported. This is because the generated Symbol is a primitive type value, not an object.

The Symbol function can accept a string as a parameter, representing a description of the Symbol instance

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