Looping Through Objects with for ... in

Question

javascript beginner. "The for...in" question.

Applied to an array object, but days cannot be defined.

<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Loops and Control</title>
</head>

<body>
<script>
  var days = ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
  var message = "";
  for (i in days) {
  message += 'Day ' + i + ' is' 
  day[i] + '\n';
  }
 alert(message);
 </script>
</body>
</html>

What exactly is your question? You are missing a `+` and `day` should be `days`. Your browser's console should tell you what is wrong. — Felix Kling, Feb 19 '17 at 08:52
it is not object it is an array. and try using reduce instead of loop — user93, Feb 19 '17 at 08:53

score 1 · Answer 1 · answered Feb 19 '17 at 08:54

You should use for loop to iterate through Arrays. For in is not recommended for Arrays.

Otherwise you can also use forEach

var days = ['Sun','Mon','Tue','Wed','Thu','Fri','Sat'];
var message = [];

for(var i = 0; i < days.length; i++) {
   message.push('Day', (i + 1), 'is', days[i] + '\n');
}

alert(message.join(' '));

score 1 · Accepted Answer · edited May 23 '17 at 12:24

Several problems there:

You've tried to use day[i], but you have no day variable (it's called days).
for-in isn't meant for looping through array entries; details of your various options here.
You're missing a + after ' is'
Your code is falling prey to The Horror of Implicit Globals (that's a post on my anemic little blog) — you need to declare i.

So:

var days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
var message = "";
for (var i = 0; i < days.length; ++i) {
  message += 'Day ' + i + ' is ' + days[i] + '\n';
}
alert(message);

Alternately, array have several features that you can use to do operations on their entries. There's forEach, which loops through the entries:

var days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
var message = "";
days.forEach(function(day, i) {
  message += 'Day ' + i + ' is ' + days[i] + '\n';
});
alert(message);

...or reduce, which is meant for exactly this use case: Building up a result from the array's entries:

var days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
var message = days.reduce(function(m, day, i) {
  return m + 'Day ' + i + ' is ' + days[i] + '\n';
}, "");
alert(message);

@Micco: No worries. I missed out something, so I've added #4 above. — T.J. Crowder, Feb 19 '17 at 09:52

J. Maria · Answer 3 · 2017-02-19T09:00:02.297

0

The name of the array is misspelled inside the for. It should be:

for (i in days) {
    message += 'Day ' + i + ' is' + days[i] + '\n';
}

i will take the value of the index, and you use it to access the original array

edited Feb 19 '17 at 09:00

answered Feb 19 '17 at 08:55

J. Maria

362
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It is not recommended to use for..in for arrays – mplungjan Feb 19 '17 at 08:56

score 0 · Answer 4 · edited Feb 19 '17 at 09:00

0

You could achieve this with reduce method of the array

const days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
const newLine = (acc, curr) => acc += curr + '\n';
const message = days.reduce(newLine, '');
console.log(message)

edited Feb 19 '17 at 09:00

mplungjan

169,008
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answered Feb 19 '17 at 08:58

user93

1,866
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OP is a beginner. ES6 is not for beginners – mplungjan Feb 19 '17 at 08:59
@mplungjan.. this guy is obsessed by ramda . Walk through his answers to note that – Abdennour TOUMI Feb 20 '17 at 03:26

Looping Through Objects with for ... in

4 Answers4