void Keyword
| Site: | Saylor Academy |
| Course: | PRDV401: Introduction to JavaScript I |
| Book: |
void Keyword
|
| Printed by: | Guest user |
| Date: | Saturday, May 18, 2024, 11:22 PM |
void Keyword
The void operator evaluates the given
expression and then returns undefined.
Try It
Click here to try an example using Code Runner.
Syntax
void expression
Description
This operator allows evaluating expressions that produce a value into places where an
expression that evaluates to undefined is desired.
The void operator is often used merely to obtain the
undefined primitive value, usually using void(0) (which is
equivalent to void 0). In these cases, the global variable
undefined can be used.
It should be noted that the precedence
of the void operator should be taken into account and that
parentheses can help clarify the resolution of the expression following the
void operator:
void 2 === '2'; // (void 2) === '2', returns false void (2 === '2'); // void (2 === '2'), returns undefined
Examples
Immediately Invoked Function Expressions
When using an immediately-invoked function expression,
void can be used to force the function keyword
to be treated as an expression instead of a declaration.
void function iife() {
console.log("Executed!");
}();
// Output: "Executed!"
Executing the above function without the void keyword will result in an
Uncaught SyntaxError.
JavaScript URIs
When a browser follows a javascript: URI, it evaluates the code in the URI
and then replaces the contents of the page with the returned value, unless the returned
value is undefined. The void operator can be used to return
undefined. For example:
<a href="javascript:void(0);"> Click here to do nothing </a> <a href="javascript:void(document.body.style.backgroundColor='green');"> Click here for green background </a>
Note: javascript: pseudo protocol is discouraged over
other alternatives, such as unobtrusive event handlers.
Non-leaking Arrow Functions
Arrow functions introduce a short-hand braceless syntax that returns an expression. This can cause unintended side effects by returning the result of a function call that previously returned nothing. To be safe, when the return value of a function is not intended to be used, it can be passed to the void operator to ensure that (for example) changing APIs do not cause arrow functions' behaviors to change.
button.onclick = () => void doSomething();
This ensures the return value of doSomething changing from
undefined to true will not change the behavior of this code.