What are the value types of JavaScript?

There are 6 value types in JavaScript, namely: 1. String type; 2. Number type; 3. Boolean type; 4. Null type; 5. Undefined type; 6. Symbol type, which represents unique value.

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

In JavaScript, value types are also called basic data types, which are:

String, Number, Boolean, Null, and Undefined (Undefined), Symbol;

Tip: Symbol is a new data type introduced in ECMAScript6, which represents a unique value.

1. String type

The string (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 Representation:

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

2. Number type

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

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

Note: The number of values ​​that can be defined by the Number type is not unlimited. The Number type in JavaScript can only represent -(253 - 1) to (253 -1) value between.

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. .

3. Boolean type

The Boolean type has only two values, true (true) or false (false), which are used for comparison when making conditional judgments. Many, 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

4, Null type

Null is a special data type with only one value, which represents a "null" value, that is, there is no value, nothing. It is used to define a null object pointer.

Use the typeof operator to check the type of Null. You will find that the type of Null is Object, which means that Null actually uses a special value belonging to Object. So by assigning the variable to Null we create an empty object.

5. 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 name);     // 输出 undefined

6, Symbol type

Symbol is a new data type introduced in ECMAScript6, which 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

[Recommended learning: JavaScript advanced tutorial】

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