Javascript has a built in JSON parse for strings, which I think is what you have:

var myObject = JSON.parse("my json string");

to use this with your example would be:

var jsonData = JSON.parse(myMessage);
for (var i = 0; i < jsonData.counters.length; i++) {
    var counter = jsonData.counters[i];
    console.log(counter.counter_name);
}

Here is a working example

EDIT: There is a mistake in your use of for loop (I missed this on my first read, credit to @Evert for the spot). using a for-in loop will set the var to be the property name of the current loop, not the actual data. See my updated loop above for correct usage

IMPORTANT: the JSON.parse method wont work in old old browsers - so if you plan to make your website available through some sort of time bending internet connection, this could be a problem! If you really are interested though, here is a support chart (which ticks all my boxes).

You're still using double quotes around x.

var x = '[{"k":"1"}]';

Use JSON.parse to turn it into an array:

var array = JSON.parse(x);
console.log(array); // [{ "k": "1" }]

Thought these two operation can be done together for clean writing you can do the following.

map to add the difference between the two property in a new property called d

using Math.abs to keep the difference positive.

let teams = [{"team":"Arsenal","f":"79","a":"36"},
{"team":"Liverpool","f":"67","a":"30"},
{"team":"Manchester_U","f":"87","a":"45"},
{"team":"Newcastle","f":"74","a":"52"},
{"team":"Leeds","f":"53","a":"37"},
{"team":"Chelsea","f":"66","a":"38"},
{"team":"West_Ham","f":"48","a":"57"},
{"team":"Aston_Villa","f":"46","a":"47"},
{"team":"Tottenham","f":"49","a":"53"},
{"team":"Blackburn","f":"55","a":"51"},
{"team":"Southampton","f":"46","a":"54"},
{"team":"Middlesbrough","f":"35","a":"47"},
{"team":"Fulham","f":"36","a":"44"},
{"team":"Charlton","f":"38","a":"49"},
{"team":"Everton","f":"45","a":"57"},
{"team":"Bolton","f":"44","a":"62"},
{"team":"Sunderland","f":"29","a":"51"},
{"team":"Ipswich","f":"41","a":"64"},
{"team":"Derby","f":"33","a":"63"},
{"team":"Leicester","f":"30","a":"64"}];

teams.map((t) => t['d'] = Math.abs(t.f - t.a))

console.log(teams);

To reduce to the team that has the lowest difference you can use the below code with reduce

    let teams = [{"team":"Arsenal","f":"79","a":"36"},
    {"team":"Liverpool","f":"67","a":"30"},
    {"team":"Manchester_U","f":"87","a":"45"},
    {"team":"Newcastle","f":"74","a":"52"},
    {"team":"Leeds","f":"53","a":"37"},
    {"team":"Chelsea","f":"66","a":"38"},
    {"team":"West_Ham","f":"48","a":"57"},
    {"team":"Aston_Villa","f":"46","a":"47"},
    {"team":"Tottenham","f":"49","a":"53"},
    {"team":"Blackburn","f":"55","a":"51"},
    {"team":"Southampton","f":"46","a":"54"},
    {"team":"Middlesbrough","f":"35","a":"47"},
    {"team":"Fulham","f":"36","a":"44"},
    {"team":"Charlton","f":"38","a":"49"},
    {"team":"Everton","f":"45","a":"57"},
    {"team":"Bolton","f":"44","a":"62"},
    {"team":"Sunderland","f":"29","a":"51"},
    {"team":"Ipswich","f":"41","a":"64"},
    {"team":"Derby","f":"33","a":"63"},
    {"team":"Leicester","f":"30","a":"64"}];
    teams.map((t) => t['d'] = Math.abs(t.f - t.a))
    
    // now finding the team with the lowsest
    
    let result = teams.reduce((teamA,teamB) => teamA.d > teamB.d? teamB: teamA );

    console.log(result);

An easier method is to map to the Number object

result= stringBits.map(Number);