Convert JSON object back to String for Javascript

Question

I'm trying to write a recursive function that, if it contains "expr" as a key instead of "number" it will feed that section of the JSON back into the function. My code is the following:

var result = 0;

function calc(str) {
var obj = JSON.parse(str);
    if(obj.hasOwnProperty("expr")) {
        var temp = obj.expr;
        JSON.stringify(temp);
        calc(temp);
    }
    if (obj.op == "add") {
        var i = parseInt(obj.number);
        result += i;    
    } else if(obj.op == "subtract") {
        var i = parseInt(obj.number);
        result -= i;
    }
    return result;
}

I'm getting a Syntax Error saying "Unexpected token o in JSON at position 1." Does Stringify not reformat it back into a string? If so, how would I go about that?

My input is something like the following:

result = calc('{"op" : "add", "number" : 5}');
result = calc('{"op" : "subtract", "number" : 2}');
result = calc('{"op" : "add", "number" : 19}');
result = calc('{"op": "subtract", "expr" : {"op" : "add", "number" : 15}}');

JSON.stringify returns a string, it doesn't modify its argument. The mistake is the line above calc(temp) — jackarms
– jackarms, Commented Oct 24, 2017 at 4:27
Not: You are not converting a "JSON object" to a string. You are converting a JavaScript object to JSON. — Felix Kling
– Felix Kling, Commented Oct 24, 2017 at 4:40
@jackarms "it doesn't modify its argument." Note, JSON.stringify() can modify the argument passed to return a different result than argument as a string — guest271314
– guest271314, Commented Oct 24, 2017 at 4:47

pirs · Accepted Answer · 2017-10-24 14:11:46Z

1

Should be like this:

function calc(str) {

    var res = 0;
    var obj = (typeof str =="string") ? JSON.parse(str) : str;

    if(obj.hasOwnProperty("expr")) obj.number = calc(obj.expr);

    switch(obj.op){
      case "add":   
          res += +obj.number;
      break;
      case "subtract":
          res -= +obj.number;
      break;
    }    
    return res;
}

Input

var result = 0;
result += calc('{"op" : "add", "number" : 5}'); 
result += calc('{"op" : "subtract", "number" : 2}'); 
result += calc('{"op" : "add", "number" : 19}'); 
result += calc('{"op": "subtract", "expr" : {"op" : "add", "number" : 15}}');

edited Oct 24, 2017 at 14:11

answered Oct 24, 2017 at 4:29

pirs

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Comments

Kendra · Accepted Answer · 2017-10-24 17:38:22Z

0

I got it figured out.

var result = 0;

function calc(str) {
    var obj = JSON.parse(str);
    var i = parseInt(obj.number);
    if(obj.hasOwnProperty("expr")) {
        var temp = obj.expr;
        temp = JSON.stringify(temp);
        var i = calc(temp);
    }     
    if (obj.op == "add") {
        result += i;
    } else if(obj.op == "subtract") {
        result -= i;
    }
    return result;
}

answered Oct 24, 2017 at 17:38

Kendra

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