How to declare Constant Values in a JavaScript array

Question

Is it possible to declare constants to be used in an array in JavaScript? I tried this but I get an error saying that Undefined Reference for the constant values in the array.

Here is my original code:

module.exports = {
    ADVICE:             'advice',
    APPROVE_ANY:        'approve_any',
    APPROVE_FIRST:      'first_reply',
    APPROVE_MAJORITY:   'approve_majority',
    APPROVE_UNANIMOUS:  'approve_unanimous',
    OBJECTION:          'objection',
    GRIPE:              'gripe',
    TELL:               'tell',
    SILENT:             'silent',
    DIALOG_TYPES:       ['advice', 'approve_any', 'first_reply', 'approve_majority', 'approve_unanimous', 'objection', 'gripe', 'tell', 'silent'],
    YES:                'yes',
    NO:                 'no',
    NO_REPLY:           'no_reply',
    RESPONSE_TYPES:     ['yes', 'no', 'no_reply'],
    MOBILE_PROVIDER:    ['AT&T', 'T-Mobile', 'Verizon']
}

Here is my code snippet (refactored):

module.exports = {
    ADVICE:             'advice',
    APPROVE_ANY:        'approve_any',
    APPROVE_FIRST:      'first_reply',
    APPROVE_MAJORITY:   'approve_majority',
    APPROVE_UNANIMOUS:  'approve_unanimous',
    OBJECTION:          'objection',
    GRIPE:              'gripe',
    TELL:               'tell',
    SILENT:             'silent',
    DIALOG_TYPES:       [ADVICE, APPROVE_ANY, APPROVE_FIRST, APPROVE_MAJORITY, APPROVE_UNANIMOUS, OBJECTION, GRIPE, TELL, SILENT],
    YES:                'yes',
    NO:                 'no',
    NO_REPLY:           'no_reply',
    RESPONSE_TYPES:     [YES, NO, NO_REPLY],
    MOBILE_PROVIDER:    ['AT&T', 'T-Mobile', 'Verizon']
}

That's not an array. And no, you cannot refer to other parts of an object inside its object initializer expression. — Pointy
– Pointy, Commented Jul 14, 2021 at 23:43
Define the object members outside of the object; you can create the object programmatically by downcasing. — Dave Newton
– Dave Newton, Commented Jul 14, 2021 at 23:46
Note that your DIALOG_TYPES really wants to be a some kind of key/value mapping, not a straight array. There is nothing in your code to tell you what value DIALOG_TYPES[3] represents right now, other than "I just know", and that makes for imminent bugs later on. — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Jul 14, 2021 at 23:53

Barmar · Accepted Answer · 2021-07-14 23:47:40Z

1

Object properties are not variables. They'll become variables in the file that imports from this, because it will use destructuring to declare the variables. But you can't refer to them that way inside the object itself.

What you can do is declare these as variables in your module, then export them all.

const ADVICE =             'advice',
      APPROVE_ANY =        'approve_any',
      APPROVE_FIRST =      'first_reply',
      APPROVE_MAJORITY =   'approve_majority',
      APPROVE_UNANIMOUS =  'approve_unanimous',
      OBJECTION =          'objection',
      GRIPE =              'gripe',
      TELL =               'tell',
      SILENT =             'silent',
      DIALOG_TYPES =       [ADVICE, APPROVE_ANY, APPROVE_FIRST, APPROVE_MAJORITY, APPROVE_UNANIMOUS, OBJECTION, GRIPE, TELL, SILENT],
      YES =                'yes',
      NO =                 'no',
      NO_REPLY =           'no_reply',
      RESPONSE_TYPES =     [YES, NO, NO_REPLY],
      MOBILE_PROVIDER =    ['AT&T', 'T-Mobile', 'Verizon']

module.exports = {
    ADVICE,
    APPROVE_ANY,
    APPROVE_FIRST,
    APPROVE_MAJORITY,
    APPROVE_UNANIMOUS,
    OBJECTION,
    GRIPE,
    TELL,
    SILENT,
    DIALOG_TYPES,
    YES,
    NO,
    NO_REPLY,
    RESPONSE_TYPES,
    MOBILE_PROVIDER
}

answered Jul 14, 2021 at 23:47

Barmar

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5 Comments

Robert Over a year ago

It appears the this will be the way to go for my first refactor pass. It seems to maintain the least amount on changes that I have to hunt down in my downstream code. I am doing things in verification and switch statements like the following:

Robert Over a year ago

body('dialogType')             .exists()           .withMessage('dialogType field is required').          .isIn(constants.DIALOG_TYPES)

Robert Over a year ago

switch(poll_type) { case: constants.APPROVE_ANY }

Robert Over a year ago

I might consider @pomax answer for a more complete refactor, but I think I would have to inspect a lot more downstream code to reference the right constant value.

Barmar Over a year ago

The correct syntax is case constants.APPROVE_ANY: ...

Mike 'Pomax' Kamermans · Accepted Answer · 2021-07-14 23:56:48Z

1

Note that your code currently has zero way to tell which array position in DIALOG_TYPES maps to which "actual thing", so you probably want to solve this first, and in doing so, almost certainly make your module export a little easier/cleaner:

const DIALOGS = {
    ADVICE:             'advice',
    APPROVE_ANY:        'approve_any',
    APPROVE_FIRST:      'first_reply',
    APPROVE_MAJORITY:   'approve_majority',
    APPROVE_UNANIMOUS:  'approve_unanimous',
    OBJECTION:          'objection',
    GRIPE:              'gripe',
    TELL:               'tell',
    SILENT:             'silent',
};

const RESPONSES = {
    YES:                'yes',
    NO:                 'no',
    NO_REPLY:           'no_reply',
};

module.exports = {
  DIALOGS,
  DIALOG_TYPES: Object.values(DIALOGS),
  RESPONSES,
  RESPONSE_TYPES: Object.values(RESPONSES),
  MOBILE_PROVIDER: [
    'AT&T',
    'T-Mobile',
    'Verizon'
  ]
};

but even better: don't export DIALOG_TYPES and RESPONSE_TYPES at all, given that you can trivially get their values as array using Object.values() already. Just export DIALOGS and RESPONSES and have the downstream code decide whether they need a (temporary) values-only array for something.

answered Jul 14, 2021 at 23:56

Mike 'Pomax' Kamermans

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1 Comment

Robert Over a year ago

Thanks for your answer. I will probably go with this solution in the long run. But I need more time to inspect downstream code.

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