In JavaScript, you can use the JSON.parse() method to parse a JSON string and convert it into a JavaScript object.

Here is an example:

let jsonString = '{"name": "John", "age": 30}';
let obj = JSON.parse(jsonString);
console.log(obj.name); // "John"
console.log(obj.age); // 30

You can also use the JSON.parse() method to parse a JSON file. Here is an example:

let xhr = new XMLHttpRequest();
xhr.open('GET', 'data.json', true);
xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status === 200) {
        let data = JSON.parse(xhr.responseText);
        console.log(data);
    }
};
xhr.send();

In this example, the data.json file is loaded using the XMLHttpRequest object, and the JSON.parse() method is used to convert the JSON file into a JavaScript object.

It’s worth noting that JSON.parse() method throws a SyntaxError if the string passed to it is not a valid JSON, In this case, you can use try-catch block to handle the error.

try {
   let obj = JSON.parse(jsonString);
   console.log(obj);
} catch(error) {
   console.log("Error in JSON: "+error);
}

Also, you can use JSON.parse() to get a json string from a file by using the fetch API

fetch('data.json')
  .then(response => response.json())
  .then(data => console.log(data));

You can use any of the above method to parse a json string or file into a JavaScript object.

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