Is there any way to perform multiple assignment in JavaScript like this:
var a, b = "one", "two";
which would be equivalent to this:
var a = "one";
var b = "two";
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3If you want; var a = "one", b = "two";Volkan Yılmaz– Volkan Yılmaz2016-07-16 13:45:54 +00:00Commented Jul 16, 2016 at 13:45
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5 Answers
In ES6 you can do it this way:
var [a, b] = ["one", "two"];
The above code is ES6 notation and is called array destructuring/object destructuring (if it's an object).
You provide the array on the right-hand side of the expression and you have comma-separated variables surrounded by square brackets on the left-hand side.
The first variable maps to the first array value and so on.
6 Comments
webdeb
please add some more description to it, so that the answer is fully understandable
Rajaprabhu Aravindasamy
If you are unsure about the technical term that you used here, then read and add it in your answer. developer.mozilla.org/en/docs/Web/JavaScript/Reference/…
Piyush.kapoor
Its destructuring we can use array destructuring since we are talking about arrays
webdeb
var [a, b] = [1,2,3,4,5]
Will Chen
Will this incur any overhead from constructing the right-side array? Or are interpreters smart enough to have a fast path when the right side is an array literal?
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While you cannot do var a, b = "one", "two";, and get them assigned to each variable in the respective order, you can do:
var a, b; to initialize these variables as undefined.
You can also do var a, b = 'foo'; to assign b to 'foo', while initializing 'a' as undefined.
var a, b;
> a
--> undefined
> b
--> undefined
> var d, e;
> e
--> undefined
> f
--> Uncaught ReferenceError: f is not defined
> var c, g = 'foo';
> c
--> undefined
> g
--> "foo"
Object destructuring
looks like:
const user = {
id: 42,
is_verified: true
};
const {id, is_verified} = user;
console.log(id); // 42
console.log(is_verified); // true
You will also see this in JavaScript to import members of a module:
Import multiple exports from module. This inserts both foo and bar into the current scope.
import {foo, bar} from '/modules/my-module.js';
Comments
If you aren't absolutely married to the idea of the values being at the end of the statement, this works:
var a = "one", b = "two";
If you want to assign to variables that have already been declared, you can use the comma operator to make it a one-liner.
a = "ONE", b = "TWO";
Comments
I wanted to extend Ryan's answer - particularly when it comes to for loops. Normally one might write a for loop as:
for (var i = 0; i < arr.length; i++) {
As many contend, the interpretive nature of JS causes arr.length to be reevaluated on every loop - causing performance loss on large loops (depending on the nature of the upper-bound, e.g., a function). When performance is paramount, they suggest iterating backward:
for (var i = arr.length-1; i >= 0; i--) {
However, this approach will cause problems when order is relevant - not to mention being uncommon notation and unclear. To get around this problem, you can construct it thusly:
for (var i = 0, z = arr.length; i < z; i++) {
While not as succinct as a standard loop, it performs better whilest being easier to read than a reverse loop.
Comments
No you can not do this way. Javascript doesn't support this type of declaration
2 Comments
Morteza Tourani
I seems you live in ice age of
JS in ES6 you can use Destructuring Assignment