The old ways...

SaveTextCommand.js

function CommandBase(value) {
	this.value = value;
}

CommandBase.prototype.GetString = function() {
	return this.value;
};

function SaveTextCommand(value, prefix = 'save: ') {
	CommandBase.call( this, value );
	this.prefix = prefix;
}

// here, we make a new `SaveTextCommand.prototype`
// linked to `CommandBase.prototype`
SaveTextCommand.prototype = Object.create( CommandBase.prototype );

// Beware! Now `SaveTextCommand.prototype.constructor` is gone,
// and might need to be manually "fixed" if you're
// in the habit of relying on such properties!

SaveTextCommand.prototype.GetString = function() {
	return this.prefix + this.value;
};

var a = new SaveTextCommand( "a" );

const res = a.GetString();

console.log(res)