void
is an operator that evaluates a given expression and then returns undefined
.
Difference
In the modern browsers which supports ES5, there's no difference between using the void
operator and the undefined
value directly:
void 0 === undefined;
void 1 === undefined;
void 'Foo' === undefined;
However, in the old browsers which run ES3 engine, undefined
is a global property and can be changed.
console.log(undefined);
var undefined = 'foo';
console.log(undefined);
On the other hand, it's not possible to override the void
operator. Hence, void
is used as a replacement for undefined
to get the undefined
value in a safe way.
In ES5, it's impossible to override
undefined
because it's
set Writeable
as
false
.
Good to know
-
void
is an operator, not a function. So we don't need to wrap the expression in parentheses. void 0
is equivalent to void(0)
.
-
There are minifiers which use void 0
to shorten undefined
.
-
If you use immediately-invoked function expression (known as IIFE), void
can be used to treat the function
keyword as an expression, not a declaration.
Assume that we want to execute the following IIFE function:
function run() {
console.log('Executed');
}();
then we can either use void
void (function run() {
console.log('Executed');
})();
or wrap the function in parentheses as below:
(function run() {
console.log('Executed');
})();
-
void
can be used to avoid the side effect when using with arrow functions.
As we know, ES6 arrow functions allow to use the return value of function by omitting the braces from the function body.
const sum = (a, b) => a + b;
In some cases, we don't intend to use the return value of function, because it can lead to a different behavior.
Let's say that we are going to handle the click
event of a button:
button.onclick = () => doSomething();
It works as usual until someone changes doSomething
, and makes it return a boolean
value. In the case it returns false
, the default behavior of the click
event will be skipped, and it maybe the thing you don't want to.
Passing the result to void
will ensure that it doesn't change the behavior of arrow function regardless the result of executed functions:
button.onclick = () => void doSomething();
-
The advantage of using void
with arrow functions can be seen in modern libraries such as React, Svelte.
These libraries allow us to execute a function right after a component mounted to the DOM. For example,
React
provides useEffect
, Svelte
has onMount
.
If we return a function inside the callback, then the function will be invoked to clean up things, free the memory before the component is removed from the screen.
useEffect(() => doSomething());
It can produce bugs at run time. To avoid that, we can either use void
useEffect(() => void doSomething());
or using the braces:
useEffect(() => {
doSomething();
});
Good practice
The void
operator has been used in the URL prefixed with javascript:
.
By default, the browser will evaluate the code when following a javascript:
URI, and then replaces the contents of page with the returned value.
To prevent the default behavior, the code must return undefined
. That's why we have been seeing the following code:
<a href="javascript: void(0);" onclick="doSomething">
...
</a>
Nowadays, using javascript:
protocol isn't recommended. It can create a security issue since user can put unsanitized input in the event handler:
<a href="javascript: alert('unsanitized input')">...</a>
Starting from v16.9.0, React also
deprecates the usage of
javascript:
URLs.
See also