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JavaScript JSON Parse

JS JSON Parse

A common use of JSON is to exchange data to/from a web server in the form of (JSON) string.

The JSON parse() Method

When receiving data from a web server, the data is always a string.

Parse the data with JSON.parse(), and the data becomes a JavaScript object.

Suppose we received this text (JSON string) from a web server:

					 
        
          '{"name":"John", "age":30, "city":"New York"}'
        
      

Use the JavaScript function JSON.parse() to convert text into a JavaScript object:

					 
        
          const obj = JSON.parse('{"name":"John", "age":30, "city":"New York"}');
        
      
Make sure the text is in JSON format, or else you will get a syntax error.

Use the JavaScript object in your page:

					 
        
          alert(obj.name);
        
      
Array as JSON

When using the JSON.parse() on a JSON derived from an array, the method will return a JavaScript array, instead of a JavaScript object.

					 
        
          const text = '["Ford", "BMW", "Audi", "Fiat"]';
          const myArr = JSON.parse(text);
        
      
Exceptions

Date objects are not allowed in JSON.

If you need to include a date, write it as a string. You can convert it back into a date object later.

					 
        
          const text = '{"name":"John", "birth":"1986-12-14", "city":"New York"}';
          const obj = JSON.parse(text);
          obj.birth = new Date(obj.birth);

          document.getElementById("demo").innerHTML = obj.name + ", " + obj.birth;
        
      
The Reviver Parameter

You can use the second parameter, of the JSON.parse() function, called reviver.

The reviver parameter is a function that checks each property, before returning the value.

					 
        
          const text = '{"name":"John", "birth":"1986-12-14", "city":"New York"}';
          const obj = JSON.parse(text, function (key, value) {
            if (key == "birth") {
              return new Date(value);
            } else {
              return value;
            }
          });

          document.getElementById("demo").innerHTML = obj.name + ", " + obj.birth;
        
      
Parsing Functions

Functions are not allowed in JSON.

If you need to include a function, write it as a string.

You can convert it back into a function later:

					 
        
          const text = '{"name":"John", "age":"function () {return 30;}", "city":"New York"}';
          const obj = JSON.parse(text);
          obj.age = eval("(" + obj.age + ")");

          document.getElementById("demo").innerHTML = obj.name + ", " + obj.age();
        
      
You should avoid using functions in JSON, the functions will lose their scope, and you would have to use eval() to convert them back into functions.

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