TIL: JavaScript's void Operator
Today I discovered the void
operator
in JavaScript. It evaluates an expression but always returns undefined.
console.log(void "hello world") // prints `undefined`
It can be used on a IIFE , which usually uses parenthesis to make the function definition be interpreted as an expression and not a declaration:
void function() {
console.log("hello world")
}();
// prints "hello world"
(function() {
console.log("hello world")
})();
// prints "hello world"
function() {
console.log("hello world")
}();
// SyntaxError
This operator is also useful to ensure that an arrow function always return undefined:
// changes to the return value of `doSomething` won't affect this code
button.onclick = () => void doSomething();
Caveat
It’s important to note that this operator has a high precedence with right-to-left associativity, so you may want to use parenthesis to correctly construct some expressions:
void "hello" + " world" // parsed as: (void "hello") + " world"
// => 'undefined world'
void ("hello" + " world") // parsed as: void ("hello" + " world")
// => undefined
