use strict

Summary

Restricts the use of some potentially-harmful features of JavaScript (e.g., eval, with) and throws syntax errors for certain sloppy code.

Syntax

"use strict";

Examples

The following code causes a syntax error because in strict mode all variables must be declared with var.

// Be careful - all code used in the page/context must be strict mode conformant.
"use strict";
function testFunction(){
   var testvar = 4;
    return testvar;
}
intvar = testFunction();

The following code causes a syntax error because in strict mode all variables must be declared with var. Even though the strict mode directive (use strict;) is only added to the function, the whole code breaks because that function does not adhere to the strict mode rules.

function testFunction(){
   // Restricts strict mode to this function only.
   "use strict";
   // This will throw.
   testvar = 4;
   return testvar;
}
var hello = true;

The following code _does not_ cause a syntax error, even though the variable intvar variable is not declared with var, because the strict mode is scoped to the function only, while the non conformant code is found in a different scope (the global scope).

function testFunction(){
   // Restricts strict mode to this function only.
   "use strict";
   var testvar = 4;
   return testvar;
}

// This would have caused a syntax error to be thrown if there had been
// a "use strict"; directive at the top of the script (above the testFunction
// function). Since the directive is scoped to the function and not to the
// global scope (in which this code operates), this code runs normally.
intvar = 1;

When the expression "use strict"; is placed at the start of a script or a function body, the code contained in it is parsed under stricter rules than what the default JavaScript language allows. These rules include:

• Variables must be explicitly declared (var).
• Statements must end with a semicolon (;).
• Assigning values to a read-only property fails (Node.DOCUMENT_FRAGMENT_NODE = 9;).
• with statements are completely disallowed.

TODO: Add the complete set of strict mode rules.

Attributions

• Microsoft Developer Network: Windows Internet Explorer JavaScript reference Article