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JavaScript JavaScript Loops, Arrays and Objects Tracking Data Using Objects Accessing All of the Properties in an Object

Posted by Brandon Ashcraft
Brandon Ashcraft
10,259 Points

'Var' in 'For in' loops?

The video didn't specify using the 'var' keyword in the 'For In' loop, but all of a sudden it is needed during the challenge to pass. What gives? Are there times where 'var' is appropriate and and times when it is not? Here's the For In loop based on the video:

for (x in person) {
  console.log(x, ': ', person[x]);
}
script.js 
var shanghai = {
  population: 14.35e6,
  longitude: '31.2000 N',
  latitude: '121.5000 E',
  country: 'CHN'
};

for (var property in shanghai){
  console.log (property);
}
index.html 
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JavaScript Objects</title>
</head>
<body>
<script src="script.js"></script>
</body>
</html>

2 Answers

Mat Morris
Mat Morris
10,292 Points
Mat Morris
Mat Morris
10,292 Points

The for...in loop iterates over every property in arbitrary order. The reason it needs the var is so that it can set each property equal to your 'property' variable. It's replacing the variable with each property until it gets to the end of the array.

Here's some reading you can do on it:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

Brandon Ashcraft
Brandon Ashcraft
10,259 Points
Brandon Ashcraft
Brandon Ashcraft
10,259 Points

Thanks Mat! That makes sense, but it doesn't explain why it worked without setting the variable in the video.