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January 27, 2024

Destructuring assignment

The two most used data structures in JavaScript are Object and Array.

  • Objects allow us to create a single entity that stores data items by key.
  • Arrays allow us to gather data items into an ordered list.

However, when we pass these to a function, we may not need all of it. The function might only require certain elements or properties.

Destructuring assignment is a special syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more convenient.

Destructuring also works well with complex functions that have a lot of parameters, default values, and so on. Soon we’ll see that.

Array destructuring

Here’s an example of how an array is destructured into variables:

// we have an array with a name and surname
let arr = ["John", "Smith"]

// destructuring assignment
// sets firstName = arr[0]
// and surname = arr[1]
let [firstName, surname] = arr;

alert(firstName); // John
alert(surname);  // Smith

Now we can work with variables instead of array members.

It looks great when combined with split or other array-returning methods:

let [firstName, surname] = "John Smith".split(' ');
alert(firstName); // John
alert(surname);  // Smith

As you can see, the syntax is simple. There are several peculiar details though. Let’s see more examples to understand it better.

“Destructuring” does not mean “destructive”.

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. However, the array itself is not modified.

It’s just a shorter way to write:

// let [firstName, surname] = arr;
let firstName = arr[0];
let surname = arr[1];
Ignore elements using commas

Unwanted elements of the array can also be thrown away via an extra comma:

// second element is not needed
let [firstName, , title] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];

alert( title ); // Consul

In the code above, the second element of the array is skipped, the third one is assigned to title, and the rest of the array items are also skipped (as there are no variables for them).

Works with any iterable on the right-side

…Actually, we can use it with any iterable, not only arrays:

let [a, b, c] = "abc"; // ["a", "b", "c"]
let [one, two, three] = new Set([1, 2, 3]);

That works, because internally a destructuring assignment works by iterating over the right value. It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values.

Assign to anything at the left-side

We can use any “assignables” on the left side.

For instance, an object property:

let user = {};
[user.name, user.surname] = "John Smith".split(' ');

alert(user.name); // John
alert(user.surname); // Smith
Looping with .entries()

In the previous chapter, we saw the Object.entries(obj) method.

We can use it with destructuring to loop over the keys-and-values of an object:

let user = {
  name: "John",
  age: 30
};

// loop over the keys-and-values
for (let [key, value] of Object.entries(user)) {
  alert(`${key}:${value}`); // name:John, then age:30
}

The similar code for a Map is simpler, as it’s iterable:

let user = new Map();
user.set("name", "John");
user.set("age", "30");

// Map iterates as [key, value] pairs, very convenient for destructuring
for (let [key, value] of user) {
  alert(`${key}:${value}`); // name:John, then age:30
}
Swap variables trick

There’s a well-known trick for swapping values of two variables using a destructuring assignment:

let guest = "Jane";
let admin = "Pete";

// Let's swap the values: make guest=Pete, admin=Jane
[guest, admin] = [admin, guest];

alert(`${guest} ${admin}`); // Pete Jane (successfully swapped!)

Here we create a temporary array of two variables and immediately destructure it in swapped order.

We can swap more than two variables this way.

The rest ‘…’

Usually, if the array is longer than the list at the left, the “extra” items are omitted.

For example, here only two items are taken, and the rest is just ignored:

let [name1, name2] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];

alert(name1); // Julius
alert(name2); // Caesar
// Further items aren't assigned anywhere

If we’d like also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "...":

let [name1, name2, ...rest] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];

// rest is an array of items, starting from the 3rd one
alert(rest[0]); // Consul
alert(rest[1]); // of the Roman Republic
alert(rest.length); // 2

The value of rest is the array of the remaining array elements.

We can use any other variable name in place of rest, just make sure it has three dots before it and goes last in the destructuring assignment.

let [name1, name2, ...titles] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];
// now titles = ["Consul", "of the Roman Republic"]

Default values

If the array is shorter than the list of variables on the left, there will be no errors. Absent values are considered undefined:

let [firstName, surname] = [];

alert(firstName); // undefined
alert(surname); // undefined

If we want a “default” value to replace the missing one, we can provide it using =:

// default values
let [name = "Guest", surname = "Anonymous"] = ["Julius"];

alert(name);    // Julius (from array)
alert(surname); // Anonymous (default used)

Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided.

For instance, here we use the prompt function for two defaults:

// runs only prompt for surname
let [name = prompt('name?'), surname = prompt('surname?')] = ["Julius"];

alert(name);    // Julius (from array)
alert(surname); // whatever prompt gets

Please note: the prompt will run only for the missing value (surname).

Object destructuring

The destructuring assignment also works with objects.

The basic syntax is:

let {var1, var2} = {var1:…, var2:…}

We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like “pattern” for corresponding properties. In the simplest case, that’s a list of variable names in {...}.

For instance:

let options = {
  title: "Menu",
  width: 100,
  height: 200
};

let {title, width, height} = options;

alert(title);  // Menu
alert(width);  // 100
alert(height); // 200

Properties options.title, options.width and options.height are assigned to the corresponding variables.

The order does not matter. This works too:

// changed the order in let {...}
let {height, width, title} = { title: "Menu", height: 200, width: 100 }

The pattern on the left side may be more complex and specify the mapping between properties and variables.

If we want to assign a property to a variable with another name, for instance, make options.width go into the variable named w, then we can set the variable name using a colon:

let options = {
  title: "Menu",
  width: 100,
  height: 200
};

// { sourceProperty: targetVariable }
let {width: w, height: h, title} = options;

// width -> w
// height -> h
// title -> title

alert(title);  // Menu
alert(w);      // 100
alert(h);      // 200

The colon shows “what : goes where”. In the example above the property width goes to w, property height goes to h, and title is assigned to the same name.

For potentially missing properties we can set default values using "=", like this:

let options = {
  title: "Menu"
};

let {width = 100, height = 200, title} = options;

alert(title);  // Menu
alert(width);  // 100
alert(height); // 200

Just like with arrays or function parameters, default values can be any expressions or even function calls. They will be evaluated if the value is not provided.

In the code below prompt asks for width, but not for title:

let options = {
  title: "Menu"
};

let {width = prompt("width?"), title = prompt("title?")} = options;

alert(title);  // Menu
alert(width);  // (whatever the result of prompt is)

We also can combine both the colon and equality:

let options = {
  title: "Menu"
};

let {width: w = 100, height: h = 200, title} = options;

alert(title);  // Menu
alert(w);      // 100
alert(h);      // 200

If we have a complex object with many properties, we can extract only what we need:

let options = {
  title: "Menu",
  width: 100,
  height: 200
};

// only extract title as a variable
let { title } = options;

alert(title); // Menu

The rest pattern “…”

What if the object has more properties than we have variables? Can we take some and then assign the “rest” somewhere?

We can use the rest pattern, just like we did with arrays. It’s not supported by some older browsers (IE, use Babel to polyfill it), but works in modern ones.

It looks like this:

let options = {
  title: "Menu",
  height: 200,
  width: 100
};

// title = property named title
// rest = object with the rest of properties
let {title, ...rest} = options;

// now title="Menu", rest={height: 200, width: 100}
alert(rest.height);  // 200
alert(rest.width);   // 100
Gotcha if there’s no let

In the examples above variables were declared right in the assignment: let {…} = {…}. Of course, we could use existing variables too, without let. But there’s a catch.

This won’t work:

let title, width, height;

// error in this line
{title, width, height} = {title: "Menu", width: 200, height: 100};

The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. Such code blocks can be used to group statements, like this:

{
  // a code block
  let message = "Hello";
  // ...
  alert( message );
}

So here JavaScript assumes that we have a code block, that’s why there’s an error. We want destructuring instead.

To show JavaScript that it’s not a code block, we can wrap the expression in parentheses (...):

let title, width, height;

// okay now
({title, width, height} = {title: "Menu", width: 200, height: 100});

alert( title ); // Menu

Nested destructuring

If an object or an array contains other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.

In the code below options has another object in the property size and an array in the property items. The pattern on the left side of the assignment has the same structure to extract values from them:

let options = {
  size: {
    width: 100,
    height: 200
  },
  items: ["Cake", "Donut"],
  extra: true
};

// destructuring assignment split in multiple lines for clarity
let {
  size: { // put size here
    width,
    height
  },
  items: [item1, item2], // assign items here
  title = "Menu" // not present in the object (default value is used)
} = options;

alert(title);  // Menu
alert(width);  // 100
alert(height); // 200
alert(item1);  // Cake
alert(item2);  // Donut

All properties of options object except extra which is absent in the left part, are assigned to corresponding variables:

Finally, we have width, height, item1, item2 and title from the default value.

Note that there are no variables for size and items, as we take their content instead.

Smart function parameters

There are times when a function has many parameters, most of which are optional. That’s especially true for user interfaces. Imagine a function that creates a menu. It may have a width, a height, a title, an item list and so on.

Here’s a bad way to write such a function:

function showMenu(title = "Untitled", width = 200, height = 100, items = []) {
  // ...
}

In real-life, the problem is how to remember the order of arguments. Usually, IDEs try to help us, especially if the code is well-documented, but still… Another problem is how to call a function when most parameters are ok by default.

Like this?

// undefined where default values are fine
showMenu("My Menu", undefined, undefined, ["Item1", "Item2"])

That’s ugly. And becomes unreadable when we deal with more parameters.

Destructuring comes to the rescue!

We can pass parameters as an object, and the function immediately destructurizes them into variables:

// we pass object to function
let options = {
  title: "My menu",
  items: ["Item1", "Item2"]
};

// ...and it immediately expands it to variables
function showMenu({title = "Untitled", width = 200, height = 100, items = []}) {
  // title, items – taken from options,
  // width, height – defaults used
  alert( `${title} ${width} ${height}` ); // My Menu 200 100
  alert( items ); // Item1, Item2
}

showMenu(options);

We can also use more complex destructuring with nested objects and colon mappings:

let options = {
  title: "My menu",
  items: ["Item1", "Item2"]
};

function showMenu({
  title = "Untitled",
  width: w = 100,  // width goes to w
  height: h = 200, // height goes to h
  items: [item1, item2] // items first element goes to item1, second to item2
}) {
  alert( `${title} ${w} ${h}` ); // My Menu 100 200
  alert( item1 ); // Item1
  alert( item2 ); // Item2
}

showMenu(options);

The full syntax is the same as for a destructuring assignment:

function({
  incomingProperty: varName = defaultValue
  ...
})

Then, for an object of parameters, there will be a variable varName for the property incomingProperty, with defaultValue by default.

Please note that such destructuring assumes that showMenu() does have an argument. If we want all values by default, then we should specify an empty object:

showMenu({}); // ok, all values are default

showMenu(); // this would give an error

We can fix this by making {} the default value for the whole object of parameters:

function showMenu({ title = "Menu", width = 100, height = 200 } = {}) {
  alert( `${title} ${width} ${height}` );
}

showMenu(); // Menu 100 200

In the code above, the whole arguments object is {} by default, so there’s always something to destructurize.

Summary

  • Destructuring assignment allows for instantly mapping an object or array onto many variables.

  • The full object syntax:

    let {prop : varName = defaultValue, ...rest} = object

    This means that property prop should go into the variable varName and, if no such property exists, then the default value should be used.

    Object properties that have no mapping are copied to the rest object.

  • The full array syntax:

    let [item1 = defaultValue, item2, ...rest] = array

    The first item goes to item1; the second goes into item2, and all the rest makes the array rest.

  • It’s possible to extract data from nested arrays/objects, for that the left side must have the same structure as the right one.

Tasks

Destructuring assignment

importance: 5

We have an object:

let user = {
  name: "John",
  years: 30
};

Write the destructuring assignment that reads:

  • name property into the variable name.
  • years property into the variable age.
  • isAdmin property into the variable isAdmin (false, if no such property)

Here’s an example of the values after your assignment:

let user = { name: "John", years: 30 };

// your code to the left side:
// ... = user

alert( name ); // John
alert( age ); // 30
alert( isAdmin ); // false
let user = {
  name: "John",
  years: 30
};

let {name, years: age, isAdmin = false} = user;

alert( name ); // John
alert( age ); // 30
alert( isAdmin ); // false

The maximal salary

importance: 5

There is a salaries object:

let salaries = {
  "John": 100,
  "Pete": 300,
  "Mary": 250
};

Create the function topSalary(salaries) that returns the name of the top-paid person.

  • If salaries is empty, it should return null.
  • If there are multiple top-paid persons, return any of them.

P.S. Use Object.entries and destructuring to iterate over key/value pairs.

Open a sandbox with tests.

function topSalary(salaries) {

  let maxSalary = 0;
  let maxName = null;

  for(const [name, salary] of Object.entries(salaries)) {
    if (maxSalary < salary) {
      maxSalary = salary;
      maxName = name;
    }
  }

  return maxName;
}

Open the solution with tests in a sandbox.

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