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JavaScript Use Strict

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JavaScript Use Strict

"use strict" is a string that makes your program or function follow a strict operating context.

It means, the errors which were being ignored by the compiler, will now throw exception messages.

The purpose of "use strict" is to indicate that the code should be executed in "strict mode".

Advantage of using use strict

The following are the three main advantage of using use strict

  • Prohibits some syntax likely to be defined in future versions of ECMAScript.
  • Fixes mistakes that make it difficult for JavaScript engines to perform optimizations.
  • Eliminates some JavaScript silent errors by changing them to throw errors.

Browser Support

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use strictYesYesYesYesYesYesYesYesYesYesYesYesYesYes

Initialize Undeclared Variable

In strict mode, you cannot initialize a value to undeclared variables.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; x = 1; document.write("x = "+x); </script> </body> </html>

Delete Variable

The strict mode will throw an error when you are trying to delete a variable instead of deleting a property of an object.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; var x = 1; delete x; document.write(x); </script> </body> </html>

Duplicate parameter

Duplicating parameter name is not allowed in strict mode.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; function myFunction(x, x){ document.write("Parameter 1 = "+ x); document.write("<br>Parameter 2 = "+ x); } myFunction(1, 2); </script> </body> </html>

Octal literals

Octal literals are not allowed in strict mode.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; var x = 010; document.write(x); </script> </body> </html>

Octal Escape Sequences

Octal escape sequences are not allowed in strict mode.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; var x = "\010"; document.write(x); </script> </body> </html>

Read Only Object

Read only object properties are not allowed to change its value in strict mode.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; var bike = { company: "honda", model: "CBR 1000RR", color: "Red" }; Object.defineProperty(bike, "company", {writable: false}); bike.color="Black"; document.write(bike.color); = "Harley Davidson"; document.write(; </script> </body> </html>

eval() Function

eval() is not allowed to create variables in the scope under Strict Mode.


HTML Online Editor
<!DOCTYPE html> <html> <body> <script> "use strict"; eval("var x = 2"); document.write(x); </script> </body> </html>

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