A brief discussion on data type conversion in javascript

1. Convert other data types to number types

Number()->strict

parseFloat/parseInt ->non-strict

isNaN(value ) If the value is not of numeric type, it first calls Number to convert to numeric type and then determines whether it is a valid number

For example:

Number("12px"); ->NaN

parseInt("12px"); ->12

isNaN("12"); ->false

Rule:

1) If the Boolean type is converted to a number, true->1 false->0

2) Convert null/undefined to a number null->0 undefined->NaN 10+null= 10 10+undefined=NaN

3) To convert [] to a number, first call the toString method of the array to convert []->"", and then use the Number method to convert ""->0

2. Convert other data types to Boolean types

Boolean

!!

! First convert to Boolean type, and then invert

For example:

!!1->true

!!0->false

![]->false !![]-> true

Law:

Only 0, NaN, null, undefined, and "" are false when converted to Boolean types, and the rest of the conversions are true

Special case:

If there is only a single value in your conditional judgment, it first converts 1 to Boolean type (->true), and then judges whether it is true or false. If the true condition is established, On the contrary, the condition is not true

if (1) { }
 if ("3px" * 3) {//->条件不成立 "3px" * 3=NaN ->false
 }
 if ("3px" + 3) {//->条件成立 "3px" + 3="3px3" ->true
 }

"+" is not only a mathematical operation in JS, but also a string concatenation when it encounters a string

"-, *, /" can only be mathematical operations in JS. When encountering non-number types, they must be forced to be converted to numbers for mathematical operations

3. Compare two values ​​(== for comparison, ===No default conversion will be performed if the comparison data types are different). If the data types of the left and right sides are different, we need to follow a set of rules and perform default data type conversion

value1==value2

1) Object==Object compares the memory address stored by itself. If the memory addresses are different, they are not equal []==[]->false Each empty array is a separate memory address

2) Object == String Call the toString method to convert the object to a string []==""->true

3) Object == Boolean The object is converted to a number (toString- Number), Boolean is also converted to number (true->1 false->0)

[]==false ->true []->0 false->0 0==0 ->true

![]==false -> ![]Convert the empty array to Boolean type first and then negate it->false false==false->true

4) Object == Number Object converted to number []==0->true []==1->false

5) String == Boolean Both converted to number " 3"==true->false "1"==true->true

6) String == number String converted to number ""==0->true

7)Boolean==Number Convert Boolean to number true==1->true true==2->false

8)null==undefined ->true null===undefined - >false

null/undefined is not equal to any other value null==0->false

9)NaN==NaN ->false NaN and any value ( Including myself) are not equal

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