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General Restrictions

Strict Mode can be applied to an entire script by placing "use strict"; before any other statements in the script. Strict state is not shared across different scripts.

Strict Mode enforces the following:

Global variables cannot be declared without using a declaration keyword.


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<!DOCTYPE html><html><body><script>

"use strict";a=10;   // Reference Error

</script></body><html>

Non-writable properties cannot be deleted.


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<!DOCTYPE html><html><body><script>

"use strict";//delete Object.prototype;  // TypeErrordelete window.undefined;  // TypeError

</script></body><html>

Octal syntax is not supported.


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<!DOCTYPE html><html><body><script>

"use strict";var x = 0777;  // Syntax Error

</script></body><html>

The keyword 'with' is not supported.


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<!DOCTYPE html><html><body><script>

"use strict";var o = {a:3};with (o) var b=a;  // Syntax Error

</script></body><html>

'eval()' cannot create variables in the surrounding scope.


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<!DOCTYPE html><html><body><script>

"use strict";eval("var a=3;");console.log(a);  // ReferenceError

</script></body><html>

'implements', 'interface', 'package', 'private', 'protected', 'public', 'static', 'let', and 'yield' are reserved keywords.


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<!DOCTYPE html><html><body><script>

"use strict";//var let = 100;  // SyntaxErrorvar implements = 200;  // SyntaxError

</script></body><html>

Attempts cannot be made to extend a non-extensible object.


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<!DOCTYPE html><html><body><script>

"use strict";var o = {};Object.preventExtensions(o);o.a =10;  // TypeError

</script></body><html>

'eval' and 'arguments' cannot be bound or assigned.


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<!DOCTYPE html><html><body><script>

'use strict';var eval = 17;                                // SyntaxErrorarguments++;                               // SyntaxError++eval;                                         // SyntaxErrorvar obj = { set p(arguments) { } }; // SyntaxErrorvar eval;                                        // SyntaxErrortry { } catch (arguments) { }          // SyntaxErrorfunction x(eval) { }                        // SyntaxErrorfunction arguments() { }                // SyntaxErrorvar y = function eval() { };             // SyntaxErrorvar f = new Function('arguments',
                     "'use strict'; return 17;"); // SyntaxError

</script></body><html>