Data type classification

Basic data types

  • string
  • number
  • boolean
  • object
  • function
  • symbol

Reference data type

  • Object
  • Date
  • Array

A data type that does not contain any values

  • null
  • undefined

Conversion between data

The other data types are converted to Number

Conversion method

  • Number(val)
  • parseInt/parseFloat(val)
Number

The browser’s internal default Number conversion is being calculated

Number("1");	/ / 1
Number("");		/ / 0
Number("");	/ / 0
Number("1 2");	// NaN
Number(true);	/ / 1
Number(null);	/ / 0
Number(undefined);	// NaN
Number("12px");		// NaN
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Conclusion:

  • The Boolean value is converted to the number 1 or 0
  • Null is converted to the number 0
  • The empty string is converted to the number 0
  • Undefined becomes NaN. Any number plus NaN equals NaN
  • Anything that is not a number in a string will be converted to NaN
parseInt/parseFloat

ParseFloat: Converts a string to an integer, as the name suggests

ParseFloat: Converts a string to a float

parseInt("1.1");	/ / 1
parseFloat("1.1");	/ / 1.1
parseInt("10px");	/ / 10
parseInt("");	// NaN
parseInt(null);	// NaN
parseInt(undefined);	// NaN
parseInt("asd");	// NaN
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Conclusion:

  • Start with the first character on the left and stop at non-numbers
  • ParseFloat recognizes one more decimal point than parseFloat
  • Anything that is not a number in a string will be converted to NaN

Other data is converted to String

Conversion method

  • toString(val)
  • String(val)
String(123);	/ / "123"
String(123 + 1);	/ / "124"
(123).toString();	/ / "123"
(123 + 1).toString();	/ / "124"
[1.2].toString();	/ / 1, 2, ""
new Date().toString();	// "Wed Sep 02 2020 17:25:51 GMT+0800"
(123 + "1").toString();	/ / "1231"
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If a string or object appears to the left of the + sign, it is concatenated as string

Other data is converted to Boolean

The rules

  • Only 0, NaN, null, undefined, and empty string values will be converted to false, and the rest will be true

  • implicit

    • In condition judgment, the result of condition processing is Boolean value

  • explicit

    • ! Invert after converting to a Boolean value
    • !!!!! Take the inverse and then take the inverse
    • Boolean(val)
Boolean(undefined);	// false
Boolean(0);	// false
Boolean(NaN);	// false
Boolean("");	// false
Boolean({});	// true
!Boolean("");	// true
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+ conversion rule

+ converts variables to numbers

+"10";	/ / 10
+"asd";	// NaN
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Quickly convert time to timestamp

+new Date(a);/ / 1599039501529
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== conversion rule

Type the same

It compares addresses in heap memory

1= =1;	// true
"1"= ="1";	// true{} = = {};// false
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Different types

As long as null == undefined is true and null/undefined is not equal to any other data type

null= =undefined;	// true
undefined= ="1";	// false
null= ="1";	// false
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If the data types on both sides of the == sign are different, the data types on both sides are converted to numbers for comparison