What are the js data types? How to convert between these js data types?

What are the JS reference data types? There are six data types in JS: Undefined, Null, Boolean, Number, String and Object. The first five are simple data types, which are also basically commonly used js data types, and the last one is a complex data type. So how are these data types converted? Let’s take a look at the introduction below.

1. Declare variables

One statement, multiple variables

You can declare many variables in one statement. The statement starts with var and uses commas to separate variables:

var name="Gates", age=56, job="CEO";

The statement can also span multiple lines:

var name="Jim",
age=32,
job="CEO";

2. Explicit data type conversion

1. Convert numbers
There are three functions that can convert non-numeric values ​​into numeric values: Number(), parseInt(), parseFloat(). Number can be used for any data type, while the other two functions are specifically used to convert strings into numbers.

1.Number conversion:

The conversion rules are as follows:

1) If it is a Boolean value, true and false will be converted to 1 and 0 respectively.
2) If it is a numeric value, it is simply passed in and returned.
3) If it is a null value, return 0.
4) If it is undefined, return NaN.
5) If it is a string, follow the following rules:
· If the string only contains numbers, convert it to a decimal value;
· If If the string contains a valid floating point format, such as "1.1", convert it to the corresponding floating point value.
· If the string contains a valid hexadecimal format, such as "0xf", convert it to a decimal integer of the corresponding size;
· If If the string is empty, it is converted to 0;
· Others, it is converted to NaN.
6) If it is an object, call the valueOf() method of the object, and then convert the returned value according to the previous rules. If the result of the conversion is NaN, call the object's toString() method, and then convert the returned string value again according to the previous rules.

var num1 = Number("hello world"); //NaN
var num2 = Number("");                 //0
var num3 = Number("0011");        //11
var num4 = Number(true);            //1

2.parseInt()

Ignore spaces in front of the string until the first non-empty character is found, and subsequent non-numeric characters will be ignored; if the first character is not a numeric symbol or a negative sign, returns NaN; the decimal will be rounded (rounded down)

var num1 = parseInt("1234blue");  //1234
var num2 = parseInt(" ");                //NaN
var num3 = parseInt(22.5):            //22

3.parseFloat()
The same as parseInt, the only difference is that parseFloat can retain decimals.

2. Convert to string
You can convert other data types into strings.

1. Transformation function String()String() function follows the following transformation rules:

1) If the value has a toString() method, call the method and return the corresponding result;
2) If the value is null, return "null";
3) If the value is undefined, return "undefined";

2. ToString() method to convert

var num1 = 10;
var str1 = num1.toString();    //"10"

Note:
When calling the toString() method of a value, you can pass a parameter: the parameter of the output value. Through this value, you can output a value expressed in binary, octal, hexadecimal, or even any other valid base format. String value.

var num = 10;
num.toString(); //"10"
num.toString(2); //"1010"
num.toString(8); //"12"
num.toString(10); //"10"
num.toString(16); //"a"

3. Convert to boolean type
The literal values ​​true and false of Boolean type are case-sensitive. Any non-empty string, any non-zero numeric value, and any object can be converted to true, while empty strings, 0 and NaN, null, and undefined are false.

var mes = "hell0"；
var mesBool = Boolean(mes);  //true

3. Implicit conversion

1. Convert to number

var num = "123";
num = +num;

Addition, subtraction, multiplication, division and the remainder can make the string implicitly converted to number.

2. Convert to string

var str = 123;
str = str + "";

3. Convert to boolean

var bool = 123;
bool = !!bool;

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