Can't extract string from buffer

Question

How can I get the string extracted to the original value?

const buf = Buffer.from('Calling Mr. Jones', 'utf8');
const ob = {b: buf};

console.log(ob.b.toString());
    // outputs "Calling Mr. Jones"

const obS = JSON.stringify(ob);

const obExtracted = JSON.parse(obS);

console.log(obExtracted.b.toString());
    // outputs "[object Object]" instead of "Calling Mr. Jones"

console.log(obExtracted.b);
    /* outputs:

        {
          type: 'Buffer',
          data: [
             67,  97, 108, 108, 105, 110,
            103,  32,  77, 114,  46,  32,
             74, 111, 110, 101, 115
          ]
        }
    */

Answer 1

You can use a JSON.parse reviver function

Note: this code may be more complex than needed, but, I tried to make it flexible enough to handle types of data, not just Buffer

const reviver = (key, value) => {
    const defaultReviver = (data, type) => new globalThis[type](data);
    const revivers = {
        // new Buffer is deprecated, so use Buffer.from
        Buffer: data => Buffer.from(data), 
        //
        // add any other exceptions (I can't think of any)        
    };

    if (typeof value !== "object") {
        return value;
    }
    const keys = Object.keys(value);
    if (keys.length !== 2 || !keys.includes("type") || !keys.includes("data")) {
        return value;
    }
    // only concern ourselves with global classes with `.toJSON`
    // to be honest, perhaps Buffer is the only one
    // but, this code should be flexible
    if (typeof globalThis[value.type]?.prototype?.toJSON !== 'function') {
        return value;
    }
    try {
        const fn = revivers[value.type] || defaultReviver;
        return fn(value.data, value.type);
    } catch {
        return value;
    }
};

Part of your code with a single change

const buf = Buffer.from("Calling Mr. Jones", "utf8");
const ob = { b: buf };
const obS = JSON.stringify(ob);

const obExtracted = JSON.parse(obS, reviver); // use reviver

console.log(obExtracted.b.toString());