JS Strict Mode

use strict defines that the JavaScript code should be executed in strict mode.

1. What's Strict Mode

1. purpose

   • to indicate that the code should be executed in strict mode.
   • With strict mode, you can not, for example, use undeclared variables.

2. supported env

   • is new in JavaScript 1.8.5 (ECMAScript version 5).
   • not a statement, but a literal expression, ignored by earlier versions of JavaScript.
   • All modern browsers support running JavaScript in "Strict Mode".

3. Why need

   • makes it easier to write "secure" JavaScript.
   • Strict mode changes previously accepted "bad syntax" into real errors.
   • Example 1:
     • mistyping a variable name creates a new global variable.
     • In strict mode, this will throw an error.
   • Example 2:
     • In normal JavaScript, a developer will not receive any error feedback assigning values to non-writable properties.
     • In strict mode, any assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object, will throw an error.

2. Allowed/Not-Allowed with Strict Mode

With strict mode, you can NOT :

• cannot use undeclared variables.
• cannot delete a variable (delete x;) or a function(both expression and declararion)
  • will cause Uncaught SyntaxError: Delete of an unqualified identifier in strict mode.
  • mynote: but delete objects' property or arrays' element is still ok
• cannot duplicate parameter names for function
  • will cause Uncaught SyntaxError: Duplicate parameter name not allowed in this context
• cannot use Octal numeric literals(八進位) (var x = 010;)
• cannot use Octal escape characters (var x = "\010";)
• cannot write to a read-only property of objects
• cannot write to a get-only property
• cannot delete an undeletable property (delete Object.prototype;)
• cannot use the string eval / arguments as a variable (var eval = 3.14;)
• cannot use future reserved keywords as variables (e.g. public, static, interface, implements)
• cannot use the with statement (with (Math){x = cos(2)};)
• cannot use eval() create variables in the scope from which it was called (for security reasons)
• In function calls like f(), the this value was the global object. In strict mode, it is now undefined.

With strict mode, still allowed:

• re-declare variables

3. Declaring Strict Mode

To declare that JavaScript code should be executed in "strict mode", add below directive to the beginning of a script or a function:

• Declared at the beginning of a script, it has global scope (all code in the script will execute in strict mode):
• The "use strict" directive is only recognized at the beginning of a script or a function.

"use strict";

4. Example

Case 1: Without strict mode

x = 3.14;
console.log(x);

//********result**********
//3.14

Case 2: With strict mode

"use strict";
x = 3.14;
console.log(x);

//********result**********
//Uncaught ReferenceError: x is not defined

case 3: strict mode with function scope

x = 3.14;       // This will not cause an error.
myFunction();

function myFunction() {
   "use strict";
    y = 3.14;   // This will cause an error
}

case 4: strict mode with global scope

"use strict";
myFunction();

function myFunction() {
    y = 3.14;   // This will also cause an error because y is not declared
}

case 5: duplicate parameter function

console.log( myFunc("Hi", "Hello") );   // hello

function myFunc(p1, p1) {
  return p1;
}

"use strict";
console.log( myFunc("Hi", "Hello") );   // Uncaught SyntaxError: Duplicate parameter name not allowed in this context

function myFunc(p1, p1) {
  return p1;
}

case 6: For security reasons, eval() is not allowed to create variables in the scope from which it was called:

"use strict";

eval("var x = 2");
console.log(x);     //Uncaught ReferenceError: x is not defined