Why does false || null return a different result than null || false?
Can I safely rely on return myVar || false returning false if myVar is either null or false, but true otherwise?
All Combinations:
console.log(false || null); // null
console.log(null || false); // false
console.log(true || null); // true
console.log(null || true); //trueThe || operator in JavaScript doesn't necessarily return true or false. It's exact behavior is this:
If the first operand is truthy, it evaluates to the first operand. Otherwise, it evaluates to the second.
This works as expected given two boolean values, but as you have noticed, you can also use it with any other value. In both of your examples, the first operand is falsey. Thus, both expressions evaluate to the second operand.
Note about one way this is used: The behavior of || is often used to create default arguments:
function foo(optionalArg) {
optionalArg = optionalArg || "default!";
console.log(optionalArg);
}
foo("test");
foo();If optionalArg is omitted, its values implicitly becomes undefined. Because undefined is falsey, undefined || "default!" evaluates to "default!". Note that this style of default args can backfire if you pass a falsey value, like 0 or "". It's more robust to explicitly check for undefined. In ECMAScript 6, you can do this with a default value within the parameter list to be more readable:
function foo(optionalArg = "default!") {
console.log(optionalArg);
}
foo("test");
foo(false);
foo(undefined);
foo();
function foo(optionalArg = 'default') for far more semantic, readable code.
foo(null) and see that it will not result with defaultWhen you have || between two variables the following happens.
Example: a || b
Another way to write this is:
if (a) {
return a;
}
return b;
Or:
a ? a : b;
Essentially what you're doing is:
if (myVar) {
return myVar;
}
return false;
If the first part of your expression is truthy, you'll receive the first part. Otherwise, you'll get the second part. It's almost like a ternary.
firstPart || secondPart
firstPart ? firstPart : secondPart
falseif you want to return false then use nullish coalescing operator
false ?? null
// result: false
function foo(optionalArg) {
optionalArg = optionalArg || "default!";
console.log(optionalArg);
}
foo(my singingmonster);
foo(1b diamond);
