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Understanding Destructuring assignment In Javascript.

Understanding Destructuring assignment In Javascript.

June 18, 2022 - 12 min read

#react#three.js#vue

Introduction#

The two most used data structures in JavaScript are Object and Array. But when we pass those to a function, it might not need the entire array/object as a whole, but rather individual pieces.

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 great with complex functions that have a lot of parameters, default values, and so on.

Destructuring Array and iterables#

The general form of the syntax is: [ variable1, variable2, ..., variableN ] = array; An example of how the array is destructured into variables:

// we have an array with the name and surname
let arr = ["Henzzo", "Baba"]

// destructuring assignment
// sets firstName = arr[0]
// and surname = arr[1]
let [firstName, surname] = arr;
console.log( firstName ); // -> Henzzo
console.log( surname ); // -> Baba

Now we can work with variables instead of array members. It looks great when combined with split() or other array-returning methods:

const [firstName, surname] = "Henzzo Baba".split( ' ' ); // firstname -> Henzzo, surname -> Baba

“Destructuring” does not mean “destructive”.#

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. But the array itself is not modified. It’s just a shorter way to write:

let arr = ["Henzzo", "Baba"]
 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] = ["Henok", "Getachew", "Web", "Ninja"];
console.log( title ); // Web

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] );

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] = "Henzzo Baba".split( ' ' );
console.log( user.name ); // Henzzo

Looping with .entries()#

Object.values(obj) return an array of [key, value] pairs of an object; We can use it with destructuring to loop over keys-and-values of an object:

let user = {
  name: "Henok",
  feeling: "loved"
};

// loop over keys-and-values
for ( let [key, value] of Object.entries( user ) ) {
  console.log( `${key}:${value}` ); // -> name:Henok, then feeling:loved
}

let user = new Map();
user.set( "name", "Henok" );
user.set( "desc", "very cool" );
for ( let [key, value] of user ) {
  console.log( `${key}:${value}` ); //  -> name:Henok, then desc:very cool
}

The rest ‘…’#

If we want not just to get the first values, but also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "...": (if you want to understand rest in javascript I wrote a lovely blog about it here)

let [word1, word2, ...rest] = ["Programing", "Isn't", "About", "Syntax"];
console.log( word1 ); // Programing
console.log( word2 ); // Isn't

// Note that type of `rest` is Array.
console.log( rest[0] ); // About
console.log( rest[1] ); // Syntax
console.log( 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.

Default values#

If there are fewer values in the array than variables in the assignment, there will be no error. Absent values are considered undefined:

let [firstName, surname] = [];
console.log( firstName ); // undefined
console.log( 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"] = ["Henok"];
console.log( name ); // Henok (from array)
console.log( 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. But it will run only for the missing one:

// runs only prompt for surname
let [name = prompt( 'name?' ), surname = prompt( 'surname?' )] = ["Henok"];
alert( name ); // Henok (from array)
alert( surname ); // whatever prompt gets

Object destructuring#

The destructuring assignment also works with objects. The basic syntax is:

let { var1, var2 } = { var1:…, var2…}

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

For instance:

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

let { title, width, height } = options;
console.log( title ); // Menu
console.log( width ); // 100
console.log( 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, options.width to go into the variable named w , then we can set it 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
console.log( title ); // Menu
console.log( w ); // 100
console.log( 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.

// with function call
const requiredArg = ( argName ) =>
  throw new Error( `Checker: dude you forgot to pass something called ${argName} in your call` );

function auth( { firstName = 'unknown', password = requiredArg( 'password' ) } ){
  return {
    firstName,
    password
  }
}

auth() // Error: ...
auth( 'henok', 'incorrect' ) // {firstName: 'henok', password: 'incorrect'}

We also can combine both the colon and equality:

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

console.log( title ); // Menu
console.log( w ); // 100
console.log( h ); // 200

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}
console.log( rest.height ); // 200
console.log( 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 };

But why? The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. The same goes for const foo = (input) => {input * 2}. Such code blocks can be used to group statements, like this:

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

To show JavaScript that it’s not a code block, we can make it a part of an expression by wrapping in parentheses (...) :

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

Nested destructuring#

If an object or an array contains other objects and arrays, we can use more complex left-side patterns to extract deeper portions. In the code below options have 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:

let options = {
  size: {
    width: 100,
    height: 200
  },
  items: ["Cake", "Donut"],
  extra: true // something extra that we will not destruct
};

/// 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;

console.log( title ); // Menu
console.log( width ); // 100
console.log( height ); // 200
console.log( item1 ); // Cake
console.log( item2 ); // Donut

The whole options object except extra that was not mentioned, is assigned to corresponding variables. Note that size and items themself are not destructured.

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Finally, we have width, height, item1, item2, and title from the default value. If we have a complex object with many properties, we can extract only what we need:

// take size as a whole into a variable, ignore the rest
let { size } = options;

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, items-list, and so on. Here’s a bad way to write such 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?

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 syntax is the same as for a destructuring assignment: function({ incomingProperty: parameterName = defaultValue })

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( {} ); // fallback to default value
showMenu(); // this would give an error

We can fix this by making {} the default value for the whole destructuring thing:

//simplified parameters a bit for clarity
function showMenu( { title = "Menu", width = 100, height = 200 } = {} ){
  alert( `${title} ${width} ${height}` );
}

showMenu(); // Menu 100 200

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

Conculusion#

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

  • The object syntax: let {prop : varName = default, ...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 array syntax: let [item1 = default, item2, ...rest] = array The first item goes to item1 ; the second goes into item2 , all the rest makes the array rest .

  • For more complex cases, the left side must have the same structure as the right one.