How many primitive data types are there in es6

There are 6 primitive data types: 1. Null type, which represents an "empty" value, that is, there is no value; 2. Undefined type, which represents undefined; 3. Number type, which represents numbers (integers, Floating numbers, etc.); 4. String type, a piece of text wrapped in single quotes or double quotes; 5. Symbol type, etc.

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

There are 9 data types in es6, which can be divided into two categories:

• Original data types (6 types): String, Number ), Boolean, Null, Undefined, Symbol;

• Reference data types (3 types): Object, Array, Function.

Let’s talk about the 6 primitive data types.

Data type	Description
null	Null value means non-object
undefined	Undefined value, indicating an unassigned initialization value
number	Number, The value of mathematical operations
string	String, representing the flow of information
boolean	Boolean value , the value of logical operation
Symbol	represents a unique value

1, Null type

Null is a special data type with only one value, which represents a "null" value, that is, there is no value. What None, used to define a null object pointer.

Use the typeof operator to view the type of Null. You will find that the type of Null is Object

var a = null;
console.log(typeof a);

It means that Null actually uses one belonging to Object. special value. So by assigning the variable to Null we create an empty object.

2. Undefined type

Undefined is also a special data type with only one value, which means undefined. When we declare a variable but do not assign a value to the variable, the default value of the variable is Undefined. For example:

var num;
console.log(num);  // 输出 undefined

When you use the typeof operator to view unassigned variable types, you will find that their types are also undefined. For undeclared variables, use the typeof operator to check their types and you will find that undeclared variables are also undefined. The sample code is as follows:

var message;
console.log(typeof message);  // 输出 undefined
console.log(typeof str);     // 输出 undefined

Explanation: Similarities and differences between null and undefined Points:

Same points: Both are data types of a value; both return false when participating in judgment; both have no methods

Differences:

(a ), null is an object, and undefined is not an object;

(b), null is a keyword, undefined is not a keyword;

(c), when null is converted into a number is 0, and undefined is NaN when converted to a number;

(d), typeof null returns 'object', typeof undefined returns 'undefined';

alert(null == undefined) ;//true

3, Number type

The numerical (Number) type is used to define numerical values. JavaScript does not distinguish between integers and decimals (floating point numbers). The Number type is used to represent them uniformly, as shown in the following example:

var num1 = 123;     // 整数
var num2 = 3.14;    // 浮点数

Note : The values ​​that can be defined by the Number type are not unlimited. The Number type in JavaScript can only represent values ​​between -(253 - 1) and (253 -1).

For some very large or very small numbers, they can also be represented by scientific (exponential) notation, as shown in the following example:

var y=123e5;      // 123 乘以 10 的 5 次方，即 12300000
var z=123e-5;     // 123 乘以 10 的 -5 次方，即 0.00123

In addition, there are some special numbers in the Number type The values ​​are Infinity, -Infinity and NaN respectively, where

• Infinity: used to represent a value of positive infinity, generally refers to a number greater than 1.7976931348623157e 308;

• -Infinity: used to represent negative infinity values, generally refers to numbers less than 5e-324;

• NaN: not a numerical value (abbreviation of Not a Number) , used to represent invalid or undefined mathematical operation structures, such as 0 divided by 0.

Tip: If the result of a certain calculation exceeds the value range of the Number type in JavaScript, then the number will automatically be converted to infinity, with positive numbers being Infinity and negative numbers being -Infinity. .

There is a strange thing in number, which is not equal to itself. That is alert (NaN == NaN) // false;

But NaN is a data type, which means the return value of typeof NaN is 'number';

For For the above two features, ECMAScript provides a function isNaN(); its purpose is to determine whether the incoming value can be converted into a number, and the return value is a boolean value.

Converting an empty string to a number is 0;

To convert a value to a numeric type, you can use Number();

4. String type

The String type is a piece of text wrapped in single quotes '' or double quotes "", such as '123', "abc". It should be noted that single quotes and double quotes are different ways of defining a string and are not part of the string.

When defining a string, if the string contains quotation marks, you can use backslash\ to escape the quotation marks in the string, or choose different quotation marks from the string to define the string, as shown in the following example Display:

var str = "Let's have a cup of coffee.";  // 双引号中包含单引号
var str = 'He said "Hello" and left.';    // 单引号中包含双引号
var str = 'We\'ll never give up.';        // 使用反斜杠转义字符串中的单引号

5. Boolean type

The Boolean type has only two values, true (true) or false (false), when making conditional judgments It is widely used. In addition to directly using true or false to define Boolean type variables, you can also use some expressions to get Boolean type values, such as:

var a = true;   // 定义一个布尔值 true
var b = false;  // 定义一个布尔值 false
var c = 2 > 1;  // 表达式 2 > 1 成立，其结果为“真（true）”，所以 c 的值为布尔类型的 true
var d = 2 < 1;  // 表达式 2 < 1 不成立，其结果为“假（false）”，所以 c 的值为布尔类型的 false

Main uses of boolean type It is used for judgment;

Under normal circumstances, only these six situations will turn false: empty string, null, undefined, 0, -0, NaN

How to convert an array Two methods can be used to convert to boolean values: Boolean(), !!()

6. Symbol type

Symbol is a type introduced in ECMAScript6 A new data type that represents a unique value. Symbol type values ​​need to be generated using the Symbol() function, as shown in the following example:

var str = "123";
var sym1 = Symbol(str);
var sym2 = Symbol(str);
console.log(sym1);          // 输出 Symbol(123)
console.log(sym2);          // 输出 Symbol(123)
console.log(sym1 == sym2);  // 输出 false ：虽然 sym1 与 sym2 看起来是相同的，但实际上它们并不一样，根据 Symbol 类型的特点，sym1 和 sym2 都是独一无二的

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