What are the basic data types of js

js basic data types include: 1. Commonly used basic data types include undefined, null, number, boolean, string; 2. Reference data types are object types, such as Object, array, function, data, etc.

The operating environment of this article: Acer S40-51, javascript version 1.8.5, Windows 10 Home Chinese version

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#JavaScript data types are divided into two types: primitive types (i.e. basic data types) and object types (i.e. reference data types).

● Commonly used basic data types include undefined, null, number, boolean, and string;

● Reference data types are also object types, such as: Object, array, function, data etc.;

Detailed explanation of basic data types:

1. Number type

① The number type includes integers and floating point numbers ( Floating point values ​​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;

6. Numeric conversion:

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

parseInt(), convert the value to integer type, commonly used

parseFloat(), convert the value to floating point type

2, string type

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

② The string has the length attribute, and the length of the string can be obtained:

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

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

④ String conversion:

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

toString() method can convert a logical value into a character String, and returns 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 to a Boolean type

4, null type

① The null type is regarded as empty The object pointer has only one value, which is the null value, so 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 it later The object is used as an object. It is best to initialize the object to a null value

5, undefined type

① There is only one value, that is, undefined. If a variable is declared, but the variable is not given Initialization value, then the value of this 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 parameter that should be provided is not provided, and the parameter is equal to undefined;

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

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

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