Examples of usage of 2 exclamation points in JavaScript _javascript tips

Often meet in JavaScript code!! , this article is to analyze the usage of 2 exclamation points in JavaScript in a more in-depth instance form. Share for everyone to use for reference. The specific analysis is as follows:

JavaScript in the!! Is the logical "not", that is, the logic "not" on the basis of "not" once again. Through! or!! You can convert many types to bool types and make other judgments.

First, the application scenario: to determine whether an object exists

Suppose you have such a JSON object:

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

Need to judge whether exist, with!! That would be great.

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

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

Result: [Object:object]

If you implement the JSON Object! or!!, you can tell if 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

Second, through! or!! The practice of converting various types into bool type

1. Null ' non ' returns true

var temp = null;
Alert (temp); 

Result: 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);

Result: undefined

var temp;
alert (!temp);

Result: True

var temp;
Alert (!! temp);

Result: false

3. Return true to the "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 to Non-zero integral type "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 0 "non"

var temp = 0;
Alert (temp);

Results: 0

var temp = 0;
alert (!temp);

Result: True

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

Result: false

6. Return false to "non" of string

var temp= "AB";
Alert (temp);

Result: AB

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

Result: false

var temp= "AB";
Alert (!! temp);

Result: True

7. Return FALSE for "non" of array

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

Result: 1,2

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

Result: false

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

Result: True

It is believed that this article has some reference value for the learning of JavaScript programming.