Consider better support for arrays with coercion

[nexdrew]

Problem

Through testing of the coerce option, I noticed that the coercion function is called for each element in the array instead of once for the entire array, regardless of the array option. This makes it impossible to coerce the array-as-a-whole within a coercion function.

Here's an example:

var parse = require('yargs-parser')
var result = parse.detailed(process.argv.slice(2), {
  array: ['add'],
  coerce: {
    add: function (arg) {
      return arg.map(Number).reduce(function (x, y) {
        return x + y
      }, 0)
    }
  }
})
console.log(result)

$ node coerce.js --add 1 2 3
{ argv: { _: [], add: [ '1', '2', '3' ] },
  error: 
   TypeError: arg.map is not a function
       ... snip stack...,
  aliases: { add: [] },
  newAliases: {},
  configuration: 
   { 'short-option-groups': true,
     'camel-case-expansion': true,
     'dot-notation': true,
     'parse-numbers': true,
     'boolean-negation': true } }

Proposal

Defer coercion (at least for arrays) until after initial parsing is done

This would allow the above example to work as is, with the following result:

$ node coerce.js --add 1 2 3
{ argv: { _: [], add: 6 },
  error: null,
  aliases: { add: [] },
  newAliases: {},
  configuration: 
   { 'short-option-groups': true,
     'camel-case-expansion': true,
     'dot-notation': true,
     'parse-numbers': true,
     'boolean-negation': true } }

Note that no context is lost with this approach because the coercion can always iterate over the array of values if it wishes to coerce them individually.

A possible alternative could be to pass the existing value (from argv) to the coerce function as a second parameter, to potentially allow coercion to "build up" or "reduce to" a desired value, but I think this could lead to a lot of confusion and would argue against it.

[bcoe]

@nexdrew I like this approach 👍