Complete collection of JavaScript destructuring. Runnable demos and slides about the same topic: http://git.mikaelb.net/presentations/bartjs/destructuring

destructuring.js

// === Arrays

var [a, b] = [1, 2];
console.log(a, b);
//=> 1 2


// Use from functions, only select from pattern
var foo = () => [1, 2, 3];

var [a, b] = foo();
console.log(a, b);
// => 1 2


// Omit certain values
var [a, , b] = [1, 2, 3];
console.log(a, b);
// => 1 3


// Combine with spread/rest operator (accumulates the rest of the values)
var [a, ...b] = [1, 2, 3];
console.log(a, b);
// => 1 [ 2, 3 ]


// Fail-safe.
var [, , , a, b] = [1, 2, 3];
console.log(a, b);
// => undefined undefined


// Swap variables easily without temp
var a = 1, b = 2;
[b, a] = [a, b];
console.log(a, b);
// => 2 1


// Advance deep arrays
var [a, [b, [c, d]]] = [1, [2, [[[3, 4], 5], 6]]];
console.log("a:", a, "b:", b, "c:", c, "d:", d);
// => a: 1 b: 2 c: [ [ 3, 4 ], 5 ] d: 6


// === Objects

var {user: x} = {user: 5};
console.log(x);
// => 5


// Fail-safe
var {user: x} = {user2: 5};
console.log(x);
// => undefined


// More values
var {prop: x, prop2: y} = {prop: 5, prop2: 10};
console.log(x, y);
// => 5 10

// Short-hand syntax
var { prop, prop2} = {prop: 5, prop2: 10};
console.log(prop, prop2);
// => 5 10

// Equal to:
var { prop: prop, prop2: prop2} = {prop: 5, prop2: 10};
console.log(prop, prop2);
// => 5 10

// === Potential grammar hiccups

// Oops: This doesn't work:
var a, b;
{ a, b } = {a: 1, b: 2};

// But this does work
var a, b;
({ a, b } = {a: 1, b: 2});
console.log(a, b);
// => 1 2

// This due to the grammar in JS. 
// Starting with { implies a block scope, not an object literal. 
// () converts to an expression.

// From Harmony Wiki:
// Note that object literals cannot appear in
// statement positions, so a plain object
// destructuring assignment statement
//  { x } = y must be parenthesized either
// as ({ x } = y) or ({ x }) = y.


// === Combined destructuring of objects and arrays

// Combine objects and arrays
var {prop: x, prop2: [, y]} = {prop: 5, prop2: [10, 100]};
console.log(x, y);
// => 5 100


// === Nested object destructuring

// Deep objects
var {
  prop: x,
  prop2: {
    prop2: {
      nested: [ , , b]
    }
  }
} = { prop: "Hello", prop2: { prop2: { nested: ["a", "b", "c"]}}};
console.log(x, b);
// => Hello c


// === Combining all to make fun happen

// All well and good, can we do more? Yes!
// Using as method parameters
var foo = function ({prop: x}) {
  console.log(x);
};

foo({invalid: 1});
foo({prop: 1});
// => undefined
// => 1

// === Nested advanced examples

// Can also use with the advanced example
var foo = function ({
  prop: x,
  prop2: {
    prop2: {
      nested: b
    }
  }
}) {
  console.log(x, ...b);
};
foo({ prop: "Hello", prop2: { prop2: { nested: ["a", "b", "c"]}}});
// => Hello a b c


// === In combination with other ES2015 features.

// Computed property names
const name = 'fieldName';
const computedObject = { [name]: name }; // (where object is { 'fieldName': 'fieldName' })
const { [name]: nameValue } = computedObject;
console.log(nameValue)
// => fieldName



// === Rest and defaults
var ajax = function ({ url = "localhost", port: p = 80}, ...data) {
  console.log("Url:", url, "Port:", p, "Rest:", data);
};

ajax({ url: "someHost" }, "additional", "data", "hello");
// => Url: someHost Port: 80 Rest: [ 'additional', 'data', 'hello' ]

ajax({ }, "additional", "data", "hello");
// => Url: localhost Port: 80 Rest: [ 'additional', 'data', 'hello' ]

ajax({ });
// => Url: localhost Port: 80 Rest: []

// Doesn't work due to trying to destructure undefined
ajax();
//  => Uncaught TypeError: Cannot match against 'undefined' or 'null'

// To fix this we need to have default value for parameter in function
// Note: See the `= {}` at the end, saying default empty object if the first argument is undefined.
var ajax = ({ url: url = "localhost", port: p = 80} = {}) => {
  console.log("Url:", url, "Port:", p);
};

// Now this works.
ajax();
// => Url: localhost Port: 80

ajax({ });
// => Url: localhost Port: 80

ajax({ port: 8080 });
//  => Url: localhost Port: 8080

ajax({ url: "someHost", port: 8080 });
//  => Url: someHost Port: 8080


// === Similar to _.pluck
var users = [
  { user: "Name1" },
  { user: "Name2" },
  { user: "Name2" },
  { user: "Name3" }
];
var names = users.map( ({ user }) => user );
console.log(names);
// => [ 'Name1', 'Name2', 'Name2', 'Name3' ]


// === Usage in for..of loops
var users = [
  { user: "Name1" },
  { user: "Name2", age: 2 },
  { user: "Name2" },
  { user: "Name3", age: 4 }
];

for (let { user, age = "DEFAULT AGE" } of users) {
  console.log(user, age);
}
// => Name1 DEFAULT AGE
// => Name2 2
// => Name2 DEFAULT AGE
// => Name3 4

Hi @mikaelbr can I ask something?
I tried example
var {prop: x, prop2: [, y]} = {prop: 5, prop2: [10, 100]};
and when I print prop2 I get 10 instead of 100. Can you explain me why ? It shouldn't be [100] (an array containing number 100)? Since we ignore the 1st number.

Thank you in advance

Additionally in the last example
[for ({ user, age = "DEFAULT AGE" } of users) console.log(user, age)];
I don't understand why we need the brackets [].

I tried it without brackets and works fine

Hi @ariel2023. Sorry for the late response, but I don't get notifications from gists. I don't understand what you mean about the first example you get:

var { prop: x, prop2: [, y] } = { prop: 5, prop2: [10, 100] };

Here you do "name prop x" and "name the second element of prop2 y", and if you try that in e.g. Chrome you do in fact get x = 5 and y = 100 as expected. Try running

var { prop: x, prop2: [, y] } = { prop: 5, prop2: [10, 100]};
console.log({ x, y });
//=> Result: Object {x: 5, y: 100}

---

As for the second example, this can probably removed. The example was written at a time where list comprehensions still was thought to be included in the new standard. Now we know it wasn't and is probably not getting included anytime soon. We have other patterns, such as map, reduce etc which can achieve much of the same, depending on usage.

So writing the example using those constructs instead we could do:

var users = [
  { user: "Name1" },
  { user: "Name2", age: 2 },
  { user: "Name2" },
  { user: "Name3", age: 4 }
];
users.forEach(({ user, age = "DEFAULT AGE" }) => console.log(user, age));

---

As a sidenote: It works when you remove the [] more as a happy accident, because that is valid syntax for a for..of loop.

for ({ user, age = "DEFAULT AGE" } of users) console.log(user, age);

becomes

for ({ user, age = "DEFAULT AGE" } of users) {
  console.log(user, age);
}

Which is destructuring user, age from users each iteration and printing that. Please note, though, this wouldn't work in strict mode. As we are not declaring our variables before using them in the destructuring. For it to be valid we would have to do something like:

for (let { user, age = "DEFAULT AGE" } of users) {
  console.log(user, age);
}

I did something like this,

let str = "hello";
const toCapital = ([a, ...b]) => [a.toUpperCase(), ...b].join('');
console.log(toCapital(str)); // Hello

and it works!

@reoxb In JavaScript, you can iterate strings, as it is a sequence of characters. See for instance:

console.log("Hello"[0]); //=> 'H' 

Also you can get the iterator from a string, as you can with an array:

"Hello"[Symbol.iterator] // Returns the iterator.

e.g.

for (let l of "hello")
  console.log(l);

Dummy question
With default, we can avoid "TypeError: Cannot read property" on undefined value, it's great !!
But how to deal with null ?
With ajax function line 178 we have :
ajax(); // is's ok for undefined => Url: localhost Port: 80
ajax(null); // but not for null => error

@asicfr I'm afraid you can't by using default parameter. Technically, null is a value as it is defined. IMO, you don't really have to use null in JavaScript.

---

Now you could always write a decorator for a function to convert null to register as undefined, but I wouldn't necessarily say it's a good idea:

function undefinedNull (fn) {
  return function (...args) {
    return fn(...args.map(v => v == null ? void 0 : v));
  };
}
let add = undefinedNull(function (a = 5, b = 3) {
  return a + b;
});
add(); //=> 8
add(undefined, null); //=> 8
add(null, null); //=> 8
add(2, 2); //=> 4

I'd say avoiding null should be preferred.

yes, i think you're right.
thanks

wow nice gist!

IMO, you don't really have to use null in JavaScript

I would compare null in javascript to the human appendix. Basically either develops a rare cancer or more commonly an acute appendicitis, both of which can be cured by general surgery. anyway, yea avoid it, null is useless.

thanks for sharing :D wasn't sure about nested destructuring..now I know 👍

Please note, that now you can use variable as key:

{
  const key = 'user';
  const { [key]: x } = { user: 5 };

  console.log(x);
  // 5
}

Thank you

How would you use array destructuring in a for-of loop? I keep seeing syntax like this, but don't understand it:

it('in for-of loop', () => {
    for (var [a, b] of [[0, 1, 2]]) {}
    expect([a, b]).toEqual([1, 2])
  })

(Ignore the test suite syntax. How do I rewrite the for line to get for (var [a, b] of [[0, 1, 2]]) to equal [1, 2]?)

@gitasong There are a couple of non-destructuring grammar rules in play in your example, I'm just going to explain some of them to avoid potential misunderstandings for other potential readers.

When we iterate and use var to declare, it's hoisted to the top of the scope and the value of the last item in the list we iterate over is accessible. So if we did:

for (var a of [1, 2, 3]) {}
console.log(a); // Output: 3

As this is essentially the same as:

var a;
a = 1;
a = 2;
a = 3;
console.log(a); // Output: 3

This would not be the case if we used let or const:

for (let z of [1, 2, 3]) {}
console.log(z); // z is not defined

This explains why we can use a and b in your example even though it's outside the body of the for-of loop. Looking at the assignment/destructuring part, there are 2 parts to this: the destructuring in assignment part of the loop, and the destructuring of values it self.

for (var [a, b] of [[0, 1, 2]])

Says for each iteration, take the current item (in this case it's only one item, [0, 1, 2]) and destructure to a and b. If the list had additional values, it would be re-assigned accordingly. If we were to write the iteration as 1 by 1 imperative statements we would write:

// instead of something like
for (var [a, b] of [[0, 1, 2], [3, 4, 5]]) console.log(a, b);

// it would be
var a, b;
[a, b] = [0, 1, 2];
console.log(a, b); // 0 1

[a, b] = [3, 4, 5];
console.log(a, b); // 3 4

From this, we can also see that the destructuring part of it is a normal array destructuring. And seeing the gist above, we can omit values in destructuring by not having an identifier.

Meaning, we can ignore the first value in the array (the 0) and only select 1 and 2:

var a, b;
[   , a, b] = [0, 1, 2];
// ^--- note the comma. Ignoring the first item

console.log(a, b); // Output: 1 2

---

This was a very long winded and maybe too detailed explanation, but I hope it gives somewhat more insight. The short version is that you can ignore elements in destructuring by using no identifiers, and making your test pass by doing:

it('in for-of loop', () => {
  for (var [, a, b] of [[0, 1, 2]]) {}
  expect([a, b]).toEqual([1, 2]);
});

Hope this helps.

This is handy when you want to destructure and redefine. Hat-trick!

function foo(param) {
    const arg = {a, b} = param
    console.log(a, b)
    console.log(arg)
}
foo({a:1, b:2})

+1

can I set {a,b,c} to one object using pure JavaScript?

like

const {a,b,c} as params = obj; or const {a,b,c} : params = obj;

// in store.js

// long time you mustn't remember how many variables there
const obj = {a:1,b:2,c:3,d:4,e:5,f:6};

// in component

const {a,b,c} = obj;
getSomething({a,b,c});
// I know this way can do it . but long time you mustn't remember how many variables in obj
//const {d,e,f,...params} = obj;
// some day you add `y` to obj in the store and it will be in params if you don't update your component.
// getSomething(params);

@mqliutie I'm not entirely sure if I understand the question. Are you talking about constructing objects after destructuring it? Could you maybe show a more concise example?

how about:

const data = {
  valid: true,
  products: [{
    item: 'Awesome shoe',
    price: 19.99,
    inStore: ['Regent Street', 'Oxford Street', 'Harrods']
  }, {
    item: 'Fave shirt',
    price: 12.99,
    inStore: ['Regent Street']
  }]
};
const { products: [ { item, inStore: [, , x] } ]} = data;
console.log(item + " is in " + x);

why is the output "Awesome shoe is in Harrods", and not "Fav shirt is in undefined" ?

@evandrix If I understand the question correctly, the key here is the destructuring of arrays. Let's simplify the code to see only the relevant parts:

const data = [{
  item: 'Awesome shoe',
  price: 19.99,
  inStore: ['Regent Street', 'Oxford Street', 'Harrods']
}, {
  item: 'Fave shirt',
  price: 12.99,
  inStore: ['Regent Street']
}];
const [ { item, inStore: [, , x] } ] = data;
console.log(item + " is in " + x);

Which would result in the same. We can reduce the structure further if we just focus on the item and not the inStore property:

const data = [{
  item: 'Awesome shoe'
}, {
  item: 'Fave shirt'
}];
const [ { item } ] = data;
console.log(item); //=> Awesome shoe

And that again is pretty much the same as:

const data = [1, 2];
const [ item ] = data;
console.log(item); //=> 1

And hopefully it's easier to see what is going on. We're destructuring an array and fetching the first element and ignore all other in the array. As if we did const [ item, ].

Thanks! I'm using it production now. 👍

Thank you! Another one I find nifty when iterating objects and requiring both the keys and values:

Object.entries({x: "xval", y: "yval"}).map(([key, val]) => console.log(key, val));
// x xval
// y yval

Thank You!

I was looking for a way to do this but it wasn't obvious to me that this particular approach would actually work:

const {prop01, prop01: { prop10 }} = {prop00: 0, prop01: {prop10: 10, prop11: 11}}
console.log(prop01, prop10)
// => {prop10: 10, prop11: 11} 10

// Example use case:
// const { request, request: { url }} = event

... Let's simplify the code to see only the relevant parts:

const data = [{
  item: 'Awesome shoe'
}, {
  item: 'Fave shirt'
}];
const [ { item } ] = data;
console.log(item); //=> Awesome shoe

And that again is pretty much the same as:

const data = [1, 2];
const [ item ] = data;
console.log(item); //=> 1

And hopefully it's easier to see what is going on. We're destructuring an array and fetching the first element and ignore all other in the array. As if we did const [ item, ].

@mikaelbr - Very helpful, great explanation. Thank you!!!

@mikaelbr, thanks, this is fantastic... ...but I have a question. I will keep it as brief as possible, so there will be a lot of ...

• JS running in a web page reads a JSON file and uses JSON.parse(fileContent).
• Some of the JSON strings in the file can contain special sequences, e.g., \n or \t.
• When these are read in, they are preserved, right up until I pass the JSON to a class constructor and use destructuring to assign values to variables local to the object:

// ...
// myFileContents contains ..."firstArg": "split\nline",...
const myJSON = JSON.parse(myFileContents);
class myClassDoesSomeStuff {
    constructor (someArgs = {}) {
        const {
             firstArg = '',
             secondArg = '',
             // etc.
             ...
        } = someArgs;
       console.log(firstArg)
// ...
let someObject = new myClassDoesSomeStuff(myJSON);

firstArg is printed as

split
line

not as

split\nline

to get around this, I end up doing

let realFirst = JSON.stringify(firstArg).replace(/^""|""$/g, '"');

But that feels really gross, an ugly hack (but neither rude nor evil, so I can live with).

Is there a more elegant solution?

Thanks!

I realized there is an error in my "sample code", the use of console.log, which will print out the newline no matter what:

console.log(firstArg)

In my actual code, values like firstArg are being assigned to a DOM element (a text input box). Without my stringify...replace trick, the \n gets converted, but with it, it is preserved, which is what I want. Sorry for the confusion.

Hi, @PeterWhittaker! I don't know if I understand your problem correctly, but when you have a newline character like this, console.log will output it with a line break in the console of your developer tool (as you said in your second comment). But when outputting in for instance DOM you won't get two lines in your HTML output. This is as HTML is not whitespace significant (for the most part**), but if you where to inspect the element tree using your developer tool you would see an actual new line in the source code. So the newline is there, but it isn't for instance a <br> which would cause the browser to render an actual new line.

But in input textarea (note <textarea>. not <input> as this is single line) you would have new lines. With a regular <input> field I belive you would just not see the newline.

Again, I don't really understand your problem. But if you wanted to keep the actual newline character inside your input field, without using textarea. You would have to "escape it". By escaping \n you would get \\n which is not a special character but a literal text representation of newline. To escape you can do a regex replace as you've shown, but there is no need for serializing through JSON. Google "escaping newline javascript" and you'll get tons of info and insight.

We’re close! I don’t want the new line, I want the \n preserved as two characters. By doing the stringify trick, I preserve those two characters and they appear in the text field, which is what I want. Without the stringify trick, the two characters are converted to a newline, and lost as white space. So it’s working the way I want but using stringify feels like a hack - it feels like there should be a cleaner way to preserve the \n all the way through.

Read the last part of my comment above on escaping. If you want to keep \n as just literal text, you escape the new line character. Read this for instance: https://stackoverflow.com/questions/25921319/escape-new-lines-with-js

I think we're at cross-purposes a little, as I did mention that I was escaping, but was wondering whether there was a more elegant way to do it than the option I opted for - as it turns out, there isn't, really, JSON.stringify(...).replace(...) is pretty much the easiest, most robust approach, and has to be sometime, somewhere before assigning to the DOM - it isn't necessary within JS itself. The issue isn't in the JS itself, since within the JS, \n gets preserved in the string right up until it is assigned to a DOM element, as seen in https://jsfiddle.net/am392d6f/ tl;dr, when assigning to the DOM, the string must be protected against interpretation - yes, this is escaping, but not where I thought it was needed: - anElement.value = "some\nline"; //doesn't work, \n is lost - anElement.value = "some\\nline"; //works - anElement.value = (JSON.stringify("some\nline").replace(/^"|"$/g, ''); // works Thanks for your patience, I initially thought this was a problem with how I was destructuring, but destructuring, object.assign(), and straight string passing all have the same results (that's nice at least): Everything is fine within JS, the problem comes when assigning to the DOM, which is essentially an output operation, as I realize now.