How to test if a string is JSON or not?

Question

Use JSON.parse

function isJson(str) {
    try {
        JSON.parse(str);
    } catch (e) {
        return false;
    }
    return true;
}

This code is JSON.parse(1234) or JSON.parse(0) or JSON.parse(false) or JSON.parse(null) all will return true.

function isJson(str) {
    try {
        JSON.parse(str);
    } catch (e) {
        return false;
    }
    return true;
}

So I rewrote code in this way:

function isJson(item) {
    item = typeof item !== "string"
        ? JSON.stringify(item)
        : item;

    try {
        item = JSON.parse(item);
    } catch (e) {
        return false;
    }

    if (typeof item === "object" && item !== null) {
        return true;
    }

    return false;
}

Testing result:

isJson test result

Let's recap this (for 2019+).

Argument: Values such as true, false, null are valid JSON (?)

FACT: These primitive values are JSON-parsable but they are not well-formed JSON structures. JSON specification indicates JSON is built on on two structures: A collection of name/value pair (object) or an ordered list of values (array).

Argument: Exception handling shouldn't be used to do something expected.
(This is a comment that has 25+ upvotes!)

FACT: No! It's definitely legal to use try/catch, especially in a case like this. Otherwise, you'd need to do lots of string analysis stuff such as tokenizing / regex operations; which would have terrible performance.

hasJsonStructure()

This is useful if your goal is to check if some data/text has proper JSON interchange format.

function hasJsonStructure(str) {
    if (typeof str !== 'string') return false;
    try {
        const result = JSON.parse(str);
        const type = Object.prototype.toString.call(result);
        return type === '[object Object]' 
            || type === '[object Array]';
    } catch (err) {
        return false;
    }
}

Usage:

hasJsonStructure('true')             // —» false
hasJsonStructure('{"x":true}')       // —» true
hasJsonStructure('[1, false, null]') // —» true

safeJsonParse()

And this is useful if you want to be careful when parsing some data to a JavaScript value.

function safeJsonParse(str) {
    try {
        return [null, JSON.parse(str)];
    } catch (err) {
        return [err];
    }
}

Usage:

const [err, result] = safeJsonParse('[Invalid JSON}');
if (err) {
    console.log('Failed to parse JSON: ' + err.message);
} else {
    console.log(result);
}

If the server is responding with JSON then it would have an application/json content-type, if it is responding with a plain text message then it should have a text/plain content-type. Make sure the server is responding with the correct content-type and test that.