JSFiddle - React, Tailwind, and code Playground

JavaScript

//adds all new properties from the src to dst. If the property already exists, updates the number in dst. dst and src are objects
function extendNumberSet( dst, src ) {
    var allVals = [];
    for ( var i = 0; i < dst.length; i++ ) {
        allVals.push(dst[i].a);
    }
    for ( var i = 0; i < src.length; i++ ) {
        if ( allVals.indexOf( src[i].a ) === -1 ) {
            dst.push( src[i] );
        }
    }
}

var c1 = [{a:1},{a:2},{a:3}];
var c2 = [{a:1},{a:7},{a:8}];

extendNumberSet( c1, c2 );

for( var i = 0; i < c1.length; i++ ) alert( c1[i].a );