Is there a way to tell the difference between this
((a=true)=>{return a})(true)
And this?
((a=true)=>{return a})()
They both return true - how can I tell whether that true is coming from the default value of the argument, or from the value passed in to that parameter?
Is there a way to tell the difference between these two?
No. Arrow function doesn't support argument object, so there is no way to check this.
But, if you are using non-arrow functions, you can get arguments number from inside the function. For example:
(function(a=true){return arguments.length;})(); // 0
(function(a=true){return arguments.length;})(true); // 1
You can easily figure out how to extend this to multiple arguments.
arguments.length is perfectly valid in strict mode
arguments object may sometimes expose possibilities such as reading source code of currently executing function or similar things. Anyway, I agree, I don't see how that may influence finding answers to that particular question.You cannot tell the difference in your specific examples, and there is no way to tell from outside the function definitely. The only exposed info is what you explicitly expose with your return value, which is just the boolean.
From inside the function, you can tell the difference if you rewrite your logic. You could change your function from
((a=true)=>{
return a;
})(true)
to
((...args)=>{
const a = args[0] === undefined ? true : args[0];
if (args.length > 0) console.log("passed arg");
else console.log("passed no arg");
return a;
})(true)
Note that you cannot combine this with default value syntax, so if you'd have to rewrite it to use rest syntax.
Alternatively, you could use a normal function instead of an arrow, and use arguments, however that is also a potentially difficult change if your real-world case relies on the arrow-function's lexical this. e.g.
(function(a = true)=>{
if (arguments.length > 0) console.log("passed arg");
else console.log("passed no arg");
return a;
})(true)
arguments.length > 0 I'd check for arguments[0] !== undefined as that's what the default initialiser uses
foo(undefined). But it was a bit unclear.Although not ideal in every way, one solution might be:
(a=undefined) => {
let is_default = false;
if (a === undefined) {
is_default = true;
a = true;
}
return a
}
undefined is pretty useless, as that's the default default value. (The only difference is the arity of the function)
If you really want to use arrow functions, you can achieve a robust (if convoluted) result using Symbol and a wrapping IIFE
var fn = ((def) =>
(a = def) => {
if(a === def) {
console.log('defaulted');
a = true;
}
console.log('a is', a);
}
)(Symbol());
both fn() and fn(true) will result in a is true - however, fn() will also output defaulted and then set a = true - though this last step I guess doesn't have to done, depends on what the real content of the real function you really want to "detect" this in
Or, a better way as pointed out by @Bergi (I keep forgetting about the block scope in ES6+ :D )
var fn;
{
const def = Symbol();
fn = (a=def) => {
if(a === def) {
console.log('defaulted');
a = true;
}
console.log('a is', a);
};
}
var fn; { const def = Symbol(); fn = (a=def) => {…}; }?
arguments.lengthargumentsin strict mode functions is perfectly valid..lengthlike in JaromandaX's suggestion is fine in strict mode.arguments.lengthis perfectly cromulent in "strict mode", so mentioning it's unavailability would be erroneous