JSFiddle - React, Tailwind, and code Playground

HTML

<script>
    // defines the "properties" getter
    Object.defineProperty( Object.prototype, 'properties', {
        get: function () {
            var arr = [], that = this;
            Object.keys( this ).forEach( function ( name ) {
                arr.push( that[ name ] );
            });
            return arr;
        }
    });
</script>

JavaScript

var schema = {
    "Files": {
        "Lines": {
            "198": {
                "firstline": "sometext"
            }
        }
    }
}

document.write( schema.Files.Lines.properties[0].firstline );