var result = string.split('').forEach(...); Doesn't work in JavaScript

Question

In Javascript, why is result below undefined?

var x = "happy";
var result = x.split('').forEach(function(val,index,array){
   array[index] = "0";
}); 
console.log(result);

The output is:

undefined

`forEach` doesn't return anything. You'll want to use `map`. — Bergi, Jan 22 '16 at 23:25
@Bergi it's unclear that he actually wants a new array, he appears to be trying to modify it in-place. — Alnitak, Jan 22 '16 at 23:25
did the question subject get updated to add the `var result = ` bit? — Alnitak, Jan 22 '16 at 23:27
@Alnitak: Given that he just created the array using `split`, his intention seems to be getting a new array in a one-liner. Which is exactly what `map` is supposed to do. The attempt to modify anything in-place is just a side show. — Bergi, Jan 22 '16 at 23:32
Thank you all! I think my confusion came from thinking that if `split` returns something, then it doesn't matter if `forEach` returns something. The difference between `forEach` and `map` is also useful. — Gary, Jan 22 '16 at 23:32

score 2 · Answer 1 · answered Jan 22 '16 at 23:23

2

string.split('').forEach(...) works perfectly to iterate over the characters of a string.

However forEach doesn't return a value, hence result === undefined.

answered Jan 22 '16 at 23:23

Alnitak

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score 1 · Accepted Answer · answered Jan 22 '16 at 23:23

Because forEach is editing the array in place. It doesn't return an array. Your code works with this modification:

var x = "happy";
var result = x.split('');
result.forEach(function(val,index,array){
   array[index] = "0";
}); 
console.log(result);

forEach() executes the callback function once for each array element; unlike map() or reduce() it always returns the value undefined and is not chainable. The typical use case is to execute side effects at the end of a chain. - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach#Description

score 1 · Answer 3 · answered Jan 22 '16 at 23:26

1

As stated forEach does not return a value.

You can use map()

var x = "happy";
var result = x.split('').map(function(val,index,array){
   return "0";
}); 
console.log(result);

answered Jan 22 '16 at 23:26

epascarello

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score 0 · Answer 4 · answered Jan 22 '16 at 23:29

This is because forEach iterates over the array. It doesn't return anything.

Try this:

var x = "happy";
var result = [];
x.split('').forEach(function(val,index,array){
  result.push("0");
});

console.log(result);

With underscorejs you could write it with two lines:

var x = "happy";
var result = _.map(x.split(''), function(el){ return "0"; });

var result = string.split('').forEach(...); Doesn't work in JavaScript

4 Answers4