JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr2 = [{"a":1},{"b":2},{"c":3},{"d":4},{"e":5},{"f":6}, {"a":2, "b" :3}]

var defaults = {};
arr2.forEach( function( item ){ 
    Object.keys( item ).forEach( function( key ){
        defaults[ key ] = 0;
    });
});


arr2 = $.map( arr2, function( item ){
        return $.extend( {}, defaults, item ); 
    });

console.log( arr2 );