es6从数组解构为对象的语法

Question

I'm reading a file with tab-separated values, and I'd like to transform it into an array of hashes with named properties.

I've studied the MDN page on Destructuring assignment , but some of the more involved examples don't make sense to me, and I don't see a syntax that results in a single object.

Here's what I've got so far:

return File.readFile(filepath, 'utf8')
.then((fileContents) => fileContents.split('\n').map((line) => {
    // here is where I'd convert the line of tab-separated
    // text into an object with named properties

    // this is fake, broken syntax
    return ({ prop_a: [0], prop_b: [2], prop_c: [1] }) = line.split('\t');
}));

A couple things to note:

• I'm using babel with node v5. I'm willing to load additional parsing or transform plugins if necessary.
• File.readFile is a simple ES6 Promise wrapper around the node-native fs.readFile(path, opt, callback) API.

I'm looking for a single statement that can split line and arbitrarily assign from that into a newly created object. I assume destructuring is the right way to pull this off, but perhaps what's needed is eg some inventive use of rest or spread.

// sample input text
Ralphette   dog 7
Felix   cat 5

// desired output
[ { name: 'Ralphette', species: 'dog', age: '7' },
  { name: 'Felix'    , species: 'cat', age: '5' }
]

Thanks for your help!

---

ANSWER

It sounds like there's no way to do this with destructuring only. However, introducing an IIFE into the mix makes for a one-liner with less-exotic destructuring. Here's the code I used, based on @Amadan's answer:

return File.readFile(filepath, 'utf8')
.then((fileContents) => (fileContents.length === 0)
    ? []
    : fileContents
        .split('\n')
        .map((line) => (([ name, species, age ]) => ({ name, species, age }))(line.split('\t')))
)

It's quite terse, and for that reason I'd advise against using it in a real project.

If, years from now, someone discovers a way to do this without the IIFE, I hope they will post it.

Answer 1

Likely not what you want, but the closest is probably

(x => ({ prop_a: x[0], prop_b: x[2], prop_c: x[1] }))(line.split('\t'));

But it is probably easiest to just do

var parts = line.split('\t');
return { prop_a: parts[0], prop_b: parts[2], prop_c: parts[1] };

While I might be proven wrong, I don't think what you want can be done by destructuring assignment.

Answer 2

You can't destructure an array directly into an object

Instead of bundling everything together in one big messy function, this answer takes a decomposed approach. The individual functions here are easier to write, maintain, and test. Small functions that do a single task are also easier to reuse in other parts of your program.

 const lines = x => x.split("\\n") const cells = x => x.split("\\t") const makeObject = ([name, species, age]) => ({name, species, age}) const data = "Ralphette\\tdog\\t7\\nFelix\\tcat\\t5" const result = lines(data).map(x => makeObject(cells(x))) console.log(result) // [ { name: "Ralphette", species: "dog", age: "7" } // , { name: "Felix", species: "cat", age: "5" } // ] 

---

If you have a compose function, it becomes just a bit cleaner

const compose = (f,g) => x => f(g(x))

const result = lines(data).map(compose(makeObject,cells))

console.log(result)
// [ { name: "Ralphette", species: "dog", age: "7" }
// , { name: "Felix", species: "cat", age: "5" }
// ]

---

If, years from now, someone discovers a way to do this without the IIFE, I hope they will post it.

It's been a few years, so here's an edit to your answer that doesn't involve the immediately invoked function -

File
  .readFile(filepath, 'utf8')
  .then
    ( fileContents => 
        fileContents.length === 0
          ? []
          : fileContents
              .split('\n')
              .map(line => line.split('\t'))
              .map(([ name, species, age ]) => ({ name, species, age }))
    )

This is effectively doing the same thing as my original answer above, only it's less efficient because it's using two calls to map . Here's how you can connect lines , cells , and makeObject to your original code -

File
  .readfile(filepath, 'utf8')
  .then
    ( fileContents => 
        fileContents.length === 0
          ? []
          : lines(fileContents).map(line => makeObject(cells(line))
    )

---

And one more thing ...

File.readFile is a simple ES6 Promise wrapper around the node-native fs.readFile (path, opt, callback) API

Node ships with a Promise-based API for the fs module now. Instead of defining your own wrappers, you can simply -

const { readFile } = require("fs").promises

readFile(filePath, 'utf8').then(results => ...)

Answer 3

This may be another closest:

var parts = line.split('\t');
var [name, species, age] = parts;
return {name, species, age};