Common function to check if JSON attribute is undefined [duplicate]

I need to findout if the JSON variable is undefined or not. Below is the snippet of code.

var file = JSON.parse(getvar("fileJSON"));
console.log(JSON.stringify(file));
// set required variables
setvar("a",file.a);    
setvar("b",file.b);   
setvar("c",file.c);   
setvar("d",file.d);   
setvar("e",file.e);   
setvar("f",file.f);   

Below check works fine. But instead of repeating for each variable, Can you please suggest common function in javascript to acheive this check "if(typeof file.a!== 'undefined')". Please suggest

if(typeof file.a!== 'undefined')
setvar("a",file.a);  

if(typeof file.b!== 'undefined')
setvar("b",file.b);  

Below function dint work :

setvar("a",checkNull(file.a));   
function checkNull(var) {

   if(typeof var !== 'undefined'){ 
        return var;
    }
}

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