What are the basic types of JavaScript?

JavaScript contains six basic data types, namely: 1. String type; 2. Number type; 3. Boolean type; 4. Null Type; 5. Undefined type; 6. Symbol type.

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

Data types in JavaScript can be divided into two types: basic data types (value types) and reference data types.

There are six basic data types (value types): String, Number, Boolean, Null, Undefined, and 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

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 values ​​in the Number type, namely Infinity, -Infinity and NaN, among which

• Infinity: used to represent positive infinity values, generally refers to values ​​greater than 1.7976931348623157e 308 number;

• -Infinity: used to represent a value of negative infinity, generally refers to a number less than 5e-324;

• NaN: that is Not a Number (abbreviation for 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, and the positive number is Infinity, negative numbers are -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

Although sym1 and sym2 appear to be The same, but in fact they are not the same. According to the characteristics of the Symbol type, sym1 and sym2 are unique, so the final result is false

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