revision:
This syntax is an expression that makes it possible to "unpack values" from arrays, or "unpack properties" from objects, into distinct variables.
It's called "destructuring assignment", because it "destructurizes" by copying items into variables. But the array itself is not modified.
The basic syntax of array destructuring: let [identifier, identifier, ... ] = arrary;
The basic syntax of object destructuring: let { var1, var2 } = {var1:..., var2:...};
where "identifier" is the name of the property to access and "expression" should evaluate to an object;
after the destructuring, the variable identifier contains the property value.
To destructure the object into multiple properties, enumerate as many properties as you like adding commas ( , ) in-between: const { identifier1, identifier2, ..., identifierN } = expression;
Syntax examples
const [a, b] = array;
const [a, , b] = array;
const [a = aDefault, b] = array;
const [a, b, ...rest] = array;
const [a, , b, ...rest] = array;
const [a, b, ...{ pop, push }] = array;
const [a, b, ...[c, d]] = array;
const { a, b } = obj;
const { a: a1, b: b1 } = obj;
const { a: a1 = aDefault, b = bDefault } = obj;
const { a, b, ...rest } = obj;
const { a: a1, b: b1, ...rest } = obj;
let a, b, a1, b1, c, d, rest, pop, push;
[a, b] = array;
[a, , b] = array;
[a = aDefault, b] = array;
[a, b, ...rest] = array;
[a, , b, ...rest] = array;
[a, b, ...{ pop, push }] = array;
[a, b, ...[c, d]] = array;
({ a, b } = obj); // brackets are required
({ a: a1, b: b1 } = obj);
({ a: a1 = aDefault, b = bDefault } = obj);
({ a, b, ...rest } = obj);
({ a: a1, b: b1, ...rest } = obj);
An example of an array destructured into variables
<div>
<p class="spec" id="des-01"></p>
<p class="spec" id="des-02"></p>
<p class="spec" id="des-03"></p>
</div>
<script>
let arr = ["John", "Smith"]
document.getElementById("des-01").innerHTML = "original array: " + arr;
// destructuring assignment sets firstName = arr[0] and surName = arr[1]
let [firstName, surName] = arr;
document.getElementById("des-02").innerHTML = "firstName: " + firstName;
document.getElementById("des-03").innerHTML = "surName: " + surName;
</script>
Array destructuring allows to work with variables instead of array members. The syntax is simple, but there are several peculiar details though.
Unwanted elements of the array can 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
Works with any iterable on the right-side. Actually, we can use it with any iterable, not only arrays, because internally a destructuring assignment works by iterating over the right value. It's kind of syntax sugar for calling "for..of" over the value to the right of "=" and assigning the values.
let [a, b, c] = "abc"; // ["a", "b", "c"]
let [one, two, three] = new Set([1, 2, 3]);
Assign to anything at the left-side. Any "assignables" at the left side can be used. 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(). The Object.entries(obj) method can be used with destructuring to loop over keys-and-values of an object.
let user = {
name: "John",
age: 30
};
// loop over keys-and-values
for (let [key, value] of Object.entries(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!)
The rest '…' : usually, if the array is longer than the list at the left, the "extra" items are omitted. If we'd like also to gather all that follows – we can add one more parameter that gets "the rest" using three dots "...".
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
The value of "rest" is the array of the remaining array elements.
let [name1, name2, ...rest] = ["Julius", "Caesar", "Consul", "of the Roman Republic"];
// rest is array of items, starting from the 3rd one
alert(rest[0]); // Consul
alert(rest[1]); // of the Roman Republic
alert(rest.length); // 2
We can use any other variable name instead 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 at the left, there'll 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)
The basic syntax is: let {var1, var2} = {var1:…, var2:…}.
We should have an existing object at 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 {...}.
Properties "options.title", "options.width" and "options.height" are assigned to the corresponding variables.
let options = {
title: "Menu",
width: 100,
height: 200
};
let {title, width, height} = options;
alert(title); // Menu
alert(width); // 100
alert(height); // 200
The order does not matter. This works too:
// changed the order in let {...}
let {height, width, title} = { title: "Menu", height: 200, width: 100 }
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
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 "…" can be used if the object has more properties than variables.
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
If an object or an array contain other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.
In the code on the right, options has another object in the property size and an array in the property items. The pattern at 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
<script>
let introduction = ["Hello", "I" , "am", "Louis"];
document.getElementById("des-05").innerHTML = "original array: " + introduction;
let [greeting, pronoun] = introduction;
console.log(greeting);
console.log(pronoun);
document.getElementById("des-06").innerHTML = greeting;
document.getElementById("des-07").innerHTML = pronoun;
let [greeting1, pronoun1] = ["Hello", "I" , "am", "Louis"];
document.getElementById("des-08").innerHTML = "original array: " + [greeting1, pronoun1];
console.log(greeting);//"Hello"
console.log(pronoun);//"I"
document.getElementById("des-09").innerHTML = greeting1;
document.getElementById("des-10").innerHTML = pronoun1;
</script>
<script>
let person = {name: "Louis", country: "Belgium", job: "innovator"};
document.getElementById("des-11").innerHTML = `${person.name} lives in ${person.country} and is an ${person.job}.`;
let {name, country, job} = person;
console.log(name);
console.log(country);
console.log(job);
document.getElementById("des-12").innerHTML = name;
document.getElementById("des-13").innerHTML = country;
document.getElementById("des-14").innerHTML = job;
let {name1, country1, job1} = {name1: "Louis", country1: "Belgium", job1: "innovator"};
console.log(name1);
console.log(country1);
console.log(job1);
document.getElementById("des-15").innerHTML = name1;
document.getElementById("des-16").innerHTML = country1;
document.getElementById("des-17").innerHTML = job1;
</script>