Type conversion in javascript
JavaScript is a weakly typed language, variables and types have nothing to do, so sometimes we need to perform type conversion
1. Number conversion (number)
Two methods:
number： 类型转化走的是v8引擎最底层机制的转化规则
parseFloat、parseInt：是提供额外的方法转化规则
number
First convert the reference type to a string (tostring) method, and then convert the string to a number
字符串 => 数字：非有效数字转化为字符串
布尔值 => 数字 ：1或0
null => 数字 ： 0；
underfined => 数字 ：NaN，
‘ ’ => 数字 ：0
对象 => 数字：先把对象转化成为字符串，在转化成为数字
【】 => 数字 ：0
Number(10); // 10
Number('10'); // 10
Number(null); // 0
Number(''); // 0
Number(true); // 1
Number(false); // 0
Number([]); //
0 Number([1,2]); // NaN
Number('10a'); // NaN
Number(undefined); // NaN
parseint() parsefloat([va],[base]), search the string from left to right for valid numeric characters, knowing that it encounters an invalid string, stop the search, and return the found as a number,
If it is not a string, first convert it to a string and use this method
let str = '12.5px'
parseInt(str) // 12
parseFloat(str)// 12.5
parseFloat(true)// NaN
isNaN determines the number type:
If the current type is a number type, return false, otherwise return true
2. String type conversion (string)
The primitive type, using the tostring method () is what it looked like before wrapping a layer of quotation marks
数字 => 字符串：包裹一层引号。
NaN => 字符串： 'NaN' 。
true => 字符串： 'true' 。
null => 字符串： 'null' （浏览器会报错（禁止你使用）—— 通常可以进行转换）
undefined => 字符串：'undefined' （浏览器会报错（禁止你使用）—— 通常可以进行转换）
Object => 字符串： '[object,Object]' 。
The result of ordinary object conversion is "[object,object]", because the Object.prototype.toString method is not converted to a string, but is used to detect the data type.
String(123); // "123"
String(true); // "true"
String(null); // "null"（报错） String(undefined);// "undefined"（报错） String([1,2,3]) // "1,2,3"
String({}); // "[object Object]"
Three, Boolean type conversion (boolean)
1：‘ ’，
2：underfined，
3 ： NaN
4：null，
5：false，
6: 0 ，
The above 6 values are false when converted to boolean, and all other types are true
Boolean('') // false
Boolean(undefined) // false
Boolean(null) // false
Boolean(NaN) // false
Boolean(false) // false
Boolean(0) // false
Boolean({}) // true
Boolean([]) // true
Fourth, the conversion of primitive types
There are two cases for converting primitive types: converting to string types or other primitive types .
If it is already a primitive type, no conversion is required.
If you convert to a string type, call the toString() method in the built-in function.
If it is other basic types, call the valueOf() method in the built-in function.
If the returned type is not primitive, the toString() method will continue to be called.
If the original type has not been returned, an error is reported.