What is the correct method for parsing JSON that is prefixed with a loop in JavaScript?

I understand that using jQuery and other libraries can make handling responses easier, but I personally want to learn the most effective way to deal with these situations. Despite my extensive research on how to handle them properly, all I come across are explanations for why these responses exist: anti-hijacking.

So, as the title suggests, I am aware that a common method involves adding a while loop at the beginning, which can be removed using .replace(/^while\(\d*\);/, ''), although this approach feels somewhat crude and makeshift, and it only covers one potential variation.

Is there a more efficient way of handling this?

In an effort to keep things simple, consider the following example:

var oReq = new XMLHttpRequest();
oReq.addEventListener("load", function() {
    console.log(this.responseText);
});
oReq.open("GET", "http://www.example.org/example.json");
oReq.send();

Upon executing this code snippet, you may receive a response like:

while(1);{"menu":{"id":"file","value":"File","popup":{"menuitem":[{"value":"New","onclick":"CreateNewDoc()"},{"value":"Open","onclick":"OpenDoc()"},{"value":"Close","onclick":"CloseDoc()"}]}}}

Answer №1

This method seems quite basic and limited in scope, as it only addresses one potential scenario.

However, this is the appropriate approach to take. In fact, you could even simplify it further with the use of .slice(9). Servers typically do not add extraneous or dynamically generated code, instead opting for the most concise option to prevent JSON hijacking. Therefore, it's crucial to specifically handle the specific prefix utilized by the service you are interacting with.

Answer №2

In addition to utilizing the replace() method, you can also achieve the same result by applying the slice() function to remove the first 9 characters. Since while(1); consists of 9 characters, you can simply discard it and then utilize JSON.parse() to convert the remaining string into an object.

const js_string = 'while(1);{"menu":{"id":"file","value":"File","popup":{"menuitem":[{"value":"New","onclick":"CreateNewDoc()"},{"value":"Open","onclick":"OpenDoc()"},{"value":"Close","onclick":"CloseDoc()"}]}}}';
//since while(9); has 9 characters, remove it and parse the string as JSON object
console.log(JSON.parse(js_string.slice(9)));

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