Data type conversion of JS

I. data types in js
There are six types of data in Js, namely number, string, boolean, underline, null and object.

1,Number

The Number data type refers to numbers, including integer and floating-point numbers. as

    var a=1,b=2.0;

2,String

String refers to 0 or more characters included in single quotation mark or double quotation mark, which can be letters, numbers, punctuation, special characters, spaces and composition. Such as:

    var name = 'yang'
    console.log(name)  // 'yang'

3,Boolean

Boolean Boolean types only have true and false, which are used for conditional judgment. Such as:

    var flag = true;
    if(flag) {
        console.log('flag is true')
    }else {
        console.log('flag is false')
    }

4. null and undefined

Both null and undefined can mean "no", which means very similar

if (!undefined) {
  console.log('undefined is false');
}
// undefined is false

if (!null) {
  console.log('null is false');
}
// null is false

undefined == null
// true

5,object

An object is a set of key value pairs, which is an unordered composite data set.

var obj = {
  foo: 'Hello',
  bar: 'World'
};

Data type conversion

1. String conversion of any data type

String(x)

String(1)//"1"

String(true)//"true"

String(null)//"null"

String(undefined)//"undefined"

String({})//"[Object Object]"

toString(x)

(1).toString()  //"1"

true.toString() //"true"

null.toString()  //Uncaught TypeError: Cannot read property 'toString' of null
 // at <anonymous>:1:6

undefined.toString()  //Uncaught TypeError: Cannot read property 'toString' of undefined
//at <anonymous>:1:11
{}.toString()   //Uncaught SyntaxError: Unexpected token .

({}).toString()  //"[object Object]"

Special Usage

1+''  //  "1"
true+'' // 'true'
null+'' // 'null'
undefined+'' // 'undefined'
{}+'' // 0
var o = {}
o+ '' //{object Object}

2. Convert any data type to number

Number(some)
parseInt(some)
parseFloat(somw)
x-0
+x

3. Convert any data type to Boolean value

Boolean(some)
!!some

4. Five false values

0 NaN '' null undefined