Consider this code snippet
class A {
constructor() {
this._elem = 5;
}
get elem() {
return this._elem;
}
}
class B {
constructor() {
this._elem = 6;
}
}
class C extends A {
constructor() {
super();
}
}
class D {
constructor() {}
}
const a = new A();
const b = new B();
const c = new C();
const d = new D();
function foo(obj, name) {
const val = 'elem' in obj ? obj.elem : 42;
const val2 = obj.elem ? obj.elem : 42;
console.log(`For ${name} val: ${val}, val2: ${val2}`);
}
foo(a, 'a');
foo(b, 'b');
foo(c, 'c');
foo(d, 'd');I was trying to figure out if there ever could be a case where directly using obj.elem when the property elem doesn't exist on obj could lead to incorrect logic vis-à-vis doing a check for "elem" in obj and access it only if it exists and use the value. It seems like val and val2 would always be the same. Are both equivalent?
No, they are not equivalent, and this is because obj.elem ? obj.elem : 42 will execute the getter and see if its return value is truthy or falsy. If it happens to be falsy, then this expression will evaluate to 42. Yet, "elem" in obj is true independent on whether the method would return a falsy or truthy value.
Unrelated, but it is also not efficient, as obj.elem ? obj.elem : 42 will execute the getter twice when it returns a truthy value.