creating objects from variables ES6

Question

I have a var in ES5 code which looks like below.

  var options = {
  clientId : clientId
  keepAlive : keepAlive,
  clean : clean,
  reconnectPeriod : reconnectPeriod,
  will : lastWillMessage
};

I could change this to ES6 by saying

let options = {clientId,keepAlive,reconnectPeriod,lastWillMessage};

But how do I do this if I don't want to pass values for all the properties? For instance I just want to pass the clientId and the keepAlive. Rest will have the default values set by MQTT module. So how can I set values for only certain properties of this object?

Your ES6 version omits clean, and also renames lastWillMessage. — user663031
– user663031, Commented May 13, 2016 at 3:33
I don't think I understand the question...if the MQTT module will set default values for absent parameters, then can't you just leave them out? — Matt Browne
– Matt Browne, Commented May 13, 2016 at 3:39
You have not specified clean in the ES6 version, so of course it will be omitted. — user663031
– user663031, Commented May 13, 2016 at 3:39
"It" does not "automatically" do anything. "It" just does what you say. If you tell it to put clean in the ES6 object then it will. If you don't then it won't. It has no idea what properties some other module is expecting or how that other thing will handle their presence or absence. You have to read the MQTT docs to figure that out. The only thing ES6's shorthand is doing for you here is that if you omit the value of a property (the colon and whatever follows), it uses the value of the variable with the same name as the property. — user663031
– user663031, Commented May 13, 2016 at 3:43
It sounds like you're missing the basic idea of the new ES6 syntax...the only new feature relevant here is that if you have a property named foo and a variable named foo, you can just write foo instead of foo: foo. Otherwise it's like ES5 object literal syntax. — Matt Browne
– Matt Browne, Commented May 13, 2016 at 3:45

Rich Apodaca · Accepted Answer · 2016-05-13 03:34:27Z

1

I'm not sure this answers your question, but Object.assign might help:

var options = getOptionsFromMQTT();

Object.assign(options, {options, {clientID, keepAlive});
// options now uses whatever values are assigned to clientID and keepAlive

Comments

sheeldotme · Accepted Answer · 2016-05-13 03:46:17Z

1

If you pass mqtt.Client an object with a property value of undefined, it will use its default.

function createClient(streamBuilder, clientId, keepAlive, reconnectPeriod, clean, will) {
    let options = {clientId,keepAlive,reconnectPeriod, clean, will};
    mqtt.Client(streamBuilder, options);
}

For example will use mqtt's default values for every option but clientId, and keepAlive when called as follows: createClient(builder, 'a1', 0); If you do not intend to ever use a property, omit it and do not pass it as an option.

edited May 13, 2016 at 3:46

answered May 13, 2016 at 3:40

sheeldotme

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