JSFiddle - React, Tailwind, and code Playground

HTML

<html>
    <head>
    </head>
    <body>
        <div id='tmp'></div>
    </body>
</html>

JavaScript

var tmpjson = { "1_2" : "1_2" },
    x = 3,
    y = 4,
    newKey = x + '_' +  y;

tmpjson[newKey] = newKey ;

$( '#tmp' ).html( tmpjson[newKey]  );

if ( "1_2" in tmpjson ) {
 alert( "true" );
}

if ( newKey in tmpjson ) {
 alert( "true" );
}

delete tmpjson["1_2"];
alert(tmpjson["1_2"]);