1. Basic data types and reference data types

ECMAScript includes two different types of values: basic data types and reference data types.

Basic data types refer to simple data segments, and reference data types refer to objects composed of multiple values.

When we assign a variable to a variable, the first thing the parser has to confirm is whether the value is a basic type value or a reference type value.

2. Common basic data types:

Number, String, Boolean, Null and Undefined. Primitive data types are accessed by value, since the actual value held in the variable can be directly manipulated. Example:

var a = 10;

var b = a;

b = 20;

console.log(a); // 10 Value

Above, b obtains a copy of a's value. Although the values ​​of the two variables are equal, the two variables store two different basic data type values.

b just saves a copy of a. Therefore, the change of b has no effect on a.

JS has defined a total of 6 data types for us, namely: undefined, null, boolean (Boolean), number (number), string (string), object (object). (Other data types are defined in ES6). Among them, the first 5 is a simple data type, and object is a complex data type

Before understanding each data type, let’s talk about variables, in any language Variables are containers for storing information. For example: in mathematics, x = 5; then x is the container of the number 5, and x is the variable. Use the var keyword to define a variable in JS. For example: var x = 5;

At this point, we have a good understanding of what a data type is. In mathematics, numbers are also divided into integers, decimals, and negative numbers. . . There are several types, and there are clear regulations on what algorithm is suitable for each type. So in the language of JS, these 6 data types are defined, as well as the usage rules and usage scenarios of each type. Next we will look at them one by one.

Before talking about each type, we first introduce a js operator: typeof (used to detect the data type of variables).

var a;

typeof(a); //undefined

var b = 123;

typeof b ; //number

It can be seen from the above code that typeof() is optional.

undefined

There is only one value which is undefined;

Then let’s define an undefined variable first:

var a = undefined;

We know that undefined means undefined, so what is the significance of this variable? In fact, undefined is not used to define a variable, but when the variable is not initialized, the value of undefined is automatically assigned. For example: var a;At this time, a is automatically assigned the value of undefined.

Then why do this? Let's think about mathematics. When we only write an x, can we say that this x is an integer, a decimal, or a negative number? Definitely not! In the same way, if there is no data type of undefined, we define a variable var a; can we say that this variable is a number? String? Or an object? Surely not? Now that we have undefined, we can say that this a is of type undefined. This is the meaning of undefined data type.

null

There is only one value which is null;

var b = null;

From a logical perspective, null represents a null pointer object. So what is the use of null? We can use typeof null; and find that the returned value is not null, but object, haha, you liar, didn’t you say that typeof can detect data types? In fact, this exactly illustrates the use of null. If we define a variable to prepare to save the object in the future. Then we'd better initialize this variable to null.

Another thing to mention here is: undefined is derived from the null value. So

undefined == null; //true

boolean

has two values, namely: true and false;

var c = true;

var d = false;

Although boolean The type only has two values, but all data types in js can be converted to and from boolean values. The conversion rules are as follows:

Data type

---

String Non -empty string empty string

---

# Number Non -0 digital values ​​(including infinity) 0/nan

---

## Object any object null

---

                                                                                                                                                                                                 .

number

---

This type represents a number:

var f = 1234566

;

var g = 1.2356;

In addition, the number type has a special value: NaN (Not a Number)

This value Used to indicate a situation where an operand that is supposed to return a value does not return a value (so that an error is not thrown). For example, in other programming languages, dividing any numeric value by zero results in an error that stops code execution. But in JavaScript, any value divided by 0 will return NaN, so it will not affect the execution of other code.

string

Use this type to represent strings:

var s = "abcdef";

var t = "Beijing";

The string can be represented by single quotes (') or double quotes ("). As long as the variable is represented by single quotes Or expressed in double quotes, then this variable is the string . See the following examples:

var a = "true"; typeof a; //"string"var b = "5689"; typeof b; //"string"

##object

An object is a collection of data and functions

.

##var o =new Object()

;

o.name="beijing"; //Object properties

o.run = function(){}; //Object method

When you create an object, js will automatically add the following to the object instance Default properties and methods.

Constructor - holds the function used to create the current object. hasOwnProperty(propertyName) - is used to check that the given property is in the current object instance (not exists in the prototype of the instance. Among them, the property name (propertyName) as the parameter must be specified in the form of a string (for example: o.hasOwnProperty("name")) - Used to check whether the incoming object is the prototype of another object.

propertyIsEnumerable(propertyName) - used to check whether the given property can be enumerated using the for-in statement. #toString() - Returns the string representation of the object.

valueOf() - Returns the string, numerical or Boolean representation of the object.

The above are the 6 basic data types of js. In the next article, let’s learn about the mutual conversion between these 6 data types.

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