I'm trying to create a forEach()
function that takes an array and a function, then performs the function action on each element of the array. However, when trying to pass the below anonymous function, I get undefined
. I tried adding a return
to the forEach()
function after reading some other posts but then the function doesn't run at all and just returns the first array[i]
it receives without modifying it.
function forEach(array, action){
for(var i = 0; i < array.length; i++)
action(array[i]);
}
var myArray = [1, 2, 3];
var something = forEach(myArray, function(element){return element++;});
console.log(something)
//undefined
That returns undefined
.
function forEach(array, action){
for(var i = 0; i < array.length; i++)
return action(array[i]);
}
var myArray = [1, 2, 3];
var something = forEach(myArray, function(element){return element++;});
console.log(something)
//undefined
That returns 1
.
What am I missing?
(I am aware a .forEach() function exists, I am trying this as a learning exercise)
Your forEach
function doesn't return anything. It looks like you're trying your hand at a map
implementation. If so, you need to add your results to a new array and return that array.
function map(array, action) { // Renamed forEach to a more suitable name
var results = [];
for (var i = 0; i < array.length; i++) {
results.push(action(array[i]));
}
return results;
}
Your foreach
implementation is correct - it shouldn't return anything. If you want map
(ie transform an argument into a new array), you can implement reduce
first:
function reduce(array, action, result) {
forEach(array, function (elem) {
result = action(result, elem);
});
return result;
}
and then, define map
via reduce
:
function map(array, action) {
return reduce(array, function (result, elem) {
return result.concat(action(elem));
}, []);
}
Putting it all together:
function forEach(array, action) { for (var i = 0; i < array.length; i++) action(array[i]); } function reduce(array, action, result) { forEach(array, function (elem) { result = action(result, elem); }); return result; } function map(array, action) { return reduce(array, function (result, elem) { return result.concat(action(elem)); }, []); } a = map([1, 2, 3, 4], function (x) { return ++x }); document.write('<pre>' + JSON.stringify(a, 0, 3));
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