passing a function into JSON.stringify

Question

I'm currently studying javascript by following the series "you dont know js".

In the section "types & grammer", when discussing "JSON.stringify" function, the author mentioned

var a = {
  b: 42,
  c: "42",
  d: [11,22,33]
};

JSON.stringify( a, ["b","c"] ); // "{"b":42,"c":"42"}"

JSON.stringify( a, function(k,v){
  if (k !== "c") return v;
} );
// "{"b":42,"d":[11,22,33]}"
Note: In the function replacer case, the key argument k is undefined for the first call (where the a object itself is being passed in). The if statement filters out the property named "c". Stringification is recursive, so the [1,2,3] array has each of its values (1, 2, and 3) passed as v to replacer, with indexes (0, 1, and 2) as k.

So I have came out with the following code, aimed at removing the value 22 from the JSON.stringify result

var a = {
  b: 42,
  c: "42",
  d: [11, 22, 33]
};

var result = JSON.stringify(a, function(k, v) {
  //since each index 0, 1, 2 of property "c" will be passed into function as "k", 
  //therefore if "k !== 1" should filter out value 22
  if (k !== 1) {
    return v;
  }
});

console.log(result);

I was expecting the result to be "{"b":42,"c":"42","d":[11,33]}".

However, the result was instead "{"b":42,"c":"42","d":[11,22,33]}" (as you can see, the value 22 at index 1 of property c is not filtered out)

Did I miss understood what the author said? Am I missing something?

@Quentin thx, i was not focused, it is written in the question! you learn each day.. — Kaddath
– Kaddath, Commented Jul 26, 2017 at 14:44

Quentin · Accepted Answer · 2017-07-26 14:19:12Z

3

Property names are always strings. 1 !== "1". Compare to a string instead of a number.

var a = {
  b: 42,
  c: "42",
  d: [11, 22, 33]
};

var result = JSON.stringify(a, function(k, v) {
  if (k !== "1") {       // "1" not 1 here
    return v;
  }
});

console.log(result);

answered Jul 26, 2017 at 14:19

Quentin

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4 Comments

Thor Over a year ago

Hi Quentin, thank you very much for the answer. Do you mind if I ask a follow up question. I was wondering what the author meant by the key argument k is undefined for the first call (where the a object itself is being passed in). Why is the object a itself been passed to as the argument to the anonymous function? What is the purpose of such logic?

Quentin Over a year ago

I can't think of any uses off the top of my head. (Which doesn't mean there aren't any.)

Thor Over a year ago

Sometimes I just dont get how javascript works lol. Just wondering, do you happen to know where I can find the implementation (i.e. the source code) of "JSON.stringify" method? Would love to know how it is implemented internally.

Quentin Over a year ago

Who's implementation?

Ori Shalom · Accepted Answer · 2017-07-26 14:58:54Z

The key will be string representing the property name and when iterating through the array properties it will be a string representing the array indexes.

The JSON.stringify iterator work in a way that the function is being executed for all elements in the object starting from the top element itself.

For example having the object you showed:

var a = {
  b: 42,
  c: "42",
  d: [11,22,33]
};

The function supplied for the JSON.stringify will run for the following keys and values:

The all object {b: 42, c: "42", d: [11,22,33]} doesn't have a key since it the top level element, so in this case the k variable will be undefined.
The 42 value accessed by the key b.
The "42" value accessed by the key c.
The [11,22,33] value accessed by the key d.
The 11 value accessed by the key 0.
The 22 value accessed by the key 1.
The 33 value accessed by the key 2.

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