Detailed explanation of the usage examples of 2 exclamation marks in javascript

I often see the situation in javascript code. This article analyzes the usage of 2 exclamation marks in javascript in an example form in a more in-depth manner. Share it for your reference. The specific analysis is as follows:

The !! in javascript is logical "non-non", that is, "non" again on the basis of logical "non". Through! or!!, many types can be converted into bool types and other judgments can be made.

1. Application scenario: Determine whether an object exists

Suppose there is such a json object:

 { color: "#E3E3E3", "font-weight": "bold" }

It needs to be judged whether it exists, use it!! It's best.

If you just print the object, you cannot determine whether it exists:

 var temp = { color: "#A60000", "font-weight": "bold" };alert(temp);

Result: [object: Object]

If you implement a json object! or !!, you can determine whether the json object exists:

 var temp = { color: "#A60000", "font-weight": "bold" };alert(!temp);

Result: false

 var temp = { color: "#A60000", "font-weight": "bold" };alert(!!temp);

Result: true

2. Convention of converting various types into bool types through! or!

1. Return true for null's "non"

 var temp = null;alert(temp);

Results: null

 var temp = null;alert(!temp);

Result: true

 var temp = null;alert(!!temp);

Result: false

2. Return true for undefined "non"

 var temp;alert(temp);

Results: undefined

 var temp;alert(!temp);

Result: true

 var temp;alert(!!temp);

Result: false

3. Return true for "non" of an empty string

 var temp="";alert(temp);

Result: empty

 var temp="";alert(!temp);

Result: true

 var temp="";alert(!!temp);

Result: false

4. Return false for non-zero integer "non"

 var temp=1;alert(temp);

Results: 1

 var temp=1;alert(!temp);

Result: false

 var temp=1;alert(!!temp);

Result: true

5. Return true for "non" of 0

 var temp = 0;alert(temp);

Results: 0

 var temp = 0;alert(!temp);

Result: true

 var temp = 0;alert(!!temp);

Result: false

6. Return false for "non" string

 var temp="ab";alert(temp);

Results: ab

 var temp="ab";alert(!temp);

Result: false

 var temp="ab";alert(!!temp);

Result: true

7. Return false to "non" of the array

 var temp=[1,2];alert(temp);

Results: 1,2

 var temp=[1,2];alert(!temp);

Result: false

 var temp=[1,2];alert(!!temp);

Result: true

I believe that the description in this article has certain reference value for everyone's learning of JavaScript programming.