JavaScript for...in loop not working - Object property is not defined Error

JavaScript SyntaxError - A declaration in the head of a for-of loop can't have an initializer

Last Updated : 31 Jul, 2020

This JavaScript exception a declaration in the head of a for-of loop can't have an initializer occurs if the for -of loop contains an initialization expression like |for (var i = 0 of iterable)|. This is not valid initialization in for-of loops.

Message:

SyntaxError: for-of loop head declarations cannot have 
             an initializer (Edge)
SyntaxError: a declaration in the head of a for-of loop
             can't have an initializer (Firefox)
SyntaxError: for-of loop variable declaration may not have 
             an initializer. (Chrome)

Error Type:

SyntaxError

Cause of Error: The loop contains an initialization that is invalid inside for-of loops.

Example 1:

HTML

<!DOCTYPE html>
<html>
<head>
    <title>Syntax Error</title>
</head>
<body>
    <script>
        let iter = [10, 20, 30]; 
        for (let val of iter) { 
          document.write(val + "<br>"); 
        } 
    </script>
</body>
</html>

Output:

10
20
30

Example 2: This example contains an invalid initialization inside the head of for-of the loop.

HTML

<!DOCTYPE html>
<html>
<head>
    <title>Syntax Error</title>
</head>
<body>
    <script>
        let iter = [10, 20, 30]; 
        for (let val = 10 of iter) { 
              document.write(val + "<br>"); 
        } 
    </script>
</body>
</html>

Output(in console):

SyntaxError: for-of loop variable declaration may not have 
an initializer.

JavaScript for...in loop not working - Object property is not defined Error

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PranchalKatiyar

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