06 Aug JavaScript Strict Mode
The strict mode in JavaScript is the usage of strict mode in JavaScript. It is defined as the following in the JavaScript code:
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"use strict"; |
Not Allowed: Using an undeclared variable
The code executed in the strict mode can never have undeclared variables. Let us see an example:
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<!DOCTYPE html> <html> <body> <h1>Demo Heading</h1> <p>This is the strict mode.</p> <p>Press (F12) and check the error.</p> <script> "use strict"; // the strict mode // using a variable without declaring a = 5 </script> </body> </html> |
Output
On pressing F12, the exact error is visible because we have used the strict mode and used a variable without declaring:
Not Allowed: Deleting a variable
If your JavaScript code is in strict mode, then you cannot delete the variable. Let us see an example:
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<!DOCTYPE html> <html> <body> <h1>Demo Heading</h1> <p>This is the strict mode.</p> <p>Press (F12) and check the error.</p> <script> "use strict"; // the strict mode let a = 5 // trying to delete a variable delete a; </script> </body> </html> |
Output
On pressing F12, the exact error is visible because we have used the strict mode and deleted a variable:
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