I have some functions which occasionally (not always) will receive a callback and run it. Is checking if the callback is defined/function a good style or is there a better way?
Example:
function save (callback){
.....do stuff......
if(typeof callback !== 'undefined'){
callback();
};
};
Rather than make the callback optional, just assign a default and call it no matter what
const identity = x =>
x
const save (..., callback = identity) {
// ...
return callback (...)
}
When used
save (...) // callback has no effect
save (..., console.log) // console.log is used as callback
Such a style is called continuation-passing style. Here's a real example, combinations, that generates all possible combinations of an Array input
const identity = x =>
x
const None =
Symbol ()
const combinations = ([ x = None, ...rest ], callback = identity) =>
x === None
? callback ([[]])
: combinations
( rest
, combs =>
callback (combs .concat (combs .map (c => [ x, ...c ])))
)
console.log (combinations (['A', 'B', 'C']))
// [ []
// , [ 'C' ]
// , [ 'B' ]
// , [ 'B', 'C' ]
// , [ 'A' ]
// , [ 'A', 'C' ]
// , [ 'A', 'B' ]
// , [ 'A', 'B', 'C' ]
// ]Because combinations is defined in continuation-passing style, the above call is effectively the same
combinations (['A', 'B', 'C'], console.log)
// [ []
// , [ 'C' ]
// , [ 'B' ]
// , [ 'B', 'C' ]
// , [ 'A' ]
// , [ 'A', 'C' ]
// , [ 'A', 'B' ]
// , [ 'A', 'B', 'C' ]
// ]
We can also pass a custom continuation that does something else with the result
console.log (combinations (['A', 'B', 'C'], combs => combs.length))
// 8
// (8 total combinations)
Continuation-passing style can be used with surprisingly elegant results
const first = (x, y) =>
x
const fibonacci = (n, callback = first) =>
n === 0
? callback (0, 1)
: fibonacci
( n - 1
, (a, b) => callback (b, a + b)
)
console.log (fibonacci (10)) // 55
// 55 is the 10th fibonacci number
// (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...)