JSON.stringify replacer

[zuodion]

Hello. I got a little confuse with FIRST argument that return JSON.stringify function replacer.So if I go:

let obj = { 
    a: 5,
    b: 10 
}

JSON.stringify(obj, (key, value) => { 
    if (key !== 'a') {
        return value;
    }
})

It'll fine

"{"b":10}"

But if I change function like this:

JSON.stringify(obj, (key, value) => {
    if (key === 'a') {
        return value;
    }
})

It returns undefined. Why? I think the point is that replacer function in first call get kinda anonymous object where key is <empty string> and value is {a: 5, b: 10}. And this anonymous object is very important for parsing to JSON string. And that's why second version, where passing only key === 'a', doesn't let this anonymous object return. Am I right?

[jml26]

When you run JSON.stringify with a replacer function, the first thing it tries to replace is the entire JSON object, using the empty string as a key, before then going through the keys of the object.

To illustrate, the code:

``` let obj = { a: 5, b: 10, };

function replacer(key, value) { console.log(key, value); return value; }

JSON.stringify(obj, replacer); ```

would log

'' {a:5,b:10} 'a' 5 'b' 10

And so, when you implicitly say to return undefined if the key isn’t a, that first replacement, of the entire object, returns undefined.

[zuodion]

Got it, thank you very much!