A JavaScript dictionary (also known as an object) is a data structure composed of key-value pairs that can be used to store and access data. The following is an example of adding, deleting, modifying and querying JavaScript dictionaries:
1. Add operation
New key-value pairs can be added to a JavaScript dictionary in the following ways:
```javascript
// Create an empty dictionary object
var dict = {};
// add key-value pair
dict["key1"] = "value1";
dict.key2 = "value2";
```
2. Delete operation
A key-value pair in a JavaScript dictionary can be removed by:
```javascript
// Create a dictionary object
var dict = {key1: "value1", key2: "value2"};
// delete the key-value pair
delete dict.key1;
```
3. Modify operation
Key-value pairs in JavaScript dictionaries can be modified in the following ways:
```javascript
// Create a dictionary object
var dict = {key1: "value1", key2: "value2"};
// Modify the key-value pair
dict.key1 = "new_value1";
```
4. Query operation
Key-value pairs in JavaScript dictionaries can be queried in the following ways:
```javascript
// Create a dictionary object
var dict = {key1: "value1", key2: "value2"};
// query key-value pairs
console.log(dict.key1); // output "value1"
console.log(dict["key2"]); // output "value2"
```
It should be noted that the keys in JavaScript dictionaries must be of string type, and the values can be of any type. If you want to use other types of keys, you need to convert them to string type first.