What are the primitive types of JavaScript?

There are 6 primitive types of JavaScript, namely: 1. number (numeric type); 2. string (string type); 3. null; 4. undefined (undefined); 5. boolean (Boolean) Type); 6. symbol, representing a unique value.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

After JS was standardized in 1997, six built-in types were defined, including five primitive types and one reference type. ES6 added a new primitive type-----symbol. Next Let’s analyze these six built-in primitive types one by one:

The six primitive types

are:

• number
• string
• null
• undefined
• boolean
• symbol

1, number

Unlike other programming languages, all numbers in JavaScript are represented by floating point values. JavaScript uses the 64-bit floating point format defined by the IEEE-754 standard to represent numbers. However, in practice, JavaScript is based on 32-bit integers. Another thing to note is that there are countless real numbers, but JavaScript can only represent a limited number of them in the form of floating point numbers. That is, when real numbers are used in JavaScript, they are often only an approximate representation of the real value.

2, string

A string is an immutable ordered sequence of 16-bit values. Each character is usually from a Unicode character. Set, indexing of JavaScript strings starts from zero. A string literal in a JavaScript program is a character sequence enclosed by single or double quotes. In ECMAScript3, string literals must be written in one line, while in ECMAScript5, string literals can be split into several lines, but each line must end with a backslash (), and both backslashes and line terminators are used. It is not counted as a string literal.

The backslash () mentioned above has a special purpose in JavaScript strings. Adding a character after the backslash symbol no longer represents their literal meaning, such as , \n is an escape character, which represents a newline character.
In addition, in ES5, strings can be treated as read-only arrays. In addition to using the charAt() method, you can also use square brackets to access individual characters in the string:

3. null

null is a keyword in the JavaScript language. It represents a special value and is often used to describe "null value". Performing a typeof operation on null returns the string "object". This was originally a flaw in JavaScript design, but was gradually accepted by everyone and can be understood as a placeholder for object. Indicates an object that does not yet exist. It is often used to indicate that a function attempts to return an object that does not exist. Null means "no object", that is, there should be no value there.

typeof(null)    
结果： "object"

4, undefined

When the declared variable has not been initialized, the default value of the variable is undefined, which means "missing value", that is, there should be A value, but not yet defined. undefined is a predefined global variable, and its value is "undefined". In ES3, undefined is a read/write variable and can be assigned any value. This bug was fixed in ES5, where undefined is read-only. If you use the typeof operator to get an undefined type, "undefined" is returned, indicating that this value is the only member of the type.

typeof(undefined)      
结果： "undefined"

5, boolean

Boolean values ​​refer to true or false, on or off, yes or no. This type has only two values, the reserved words true and false. Any JavaScript value can be converted to a Boolean value. Some of the following types will be converted to false, undefined, null, 0, -0, NaN, "" //Empty string, all other values, including all objects (arrays) will be converted to true. Commonly used operators for Boolean values ​​include "&&", "||", and "!".

6. Symbol

Symbol is a new data type in ES6, which represents a unique value. The biggest use of Symbol is to define the unique attribute name of an object. . ES5 object attribute names are all strings, which can easily cause attribute name conflicts. If you use an object provided by someone else but want to add new methods later, the name of the new method may conflict with the existing method. Therefore, it is necessary to ensure that the name of each attribute is unique to prevent attribute name conflicts. This is why ES6 introduced Symbol.

[Recommended learning: javascript advanced tutorial]

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