Strict mode in Javascript

Refs

1. JavaScript Use Strict

What and why

• Defines that JavaScript code should be executed in “strict mode”.
• Introduced in ECMAScript 5
• Not a statement, but a literal expression, ignored by earlier versions of JavaScript.
• Strict mode can make your code more secure.

Not allowed

Undefined variable

"use strict";
y = 3.14;
// This will cause: Uncaught ReferenceError: y is not defined

Delete identifier

Reference: Delete in strict mode

Normal variables can’t be deleted with delete operator. delete is only meant for properties on an object.
delete has nothing to do with directly freeing memory. Memory management is done indirectly via breaking references.
This error only happens in strict mode code. In non-strict code, the operation just returns false.(note that testing this in chrome debug tools return true while in browser or node.js return false)

"use strict";
// variable
let x = 3.14;
delete x;     // This will cause an error: Delete of an unqualified identifier in strict mode
// function
function x(p1, p2) {}; 
delete x;   // Same as above

Octals

"use strict";
let x = 010;   // This will cause an error 

Eval to create variables

"use strict";
eval ("var x = 2");
alert (x);      // This will cause an error: Uncaught ReferenceError: x is not defined

With statement

⚠ Warning: Use of the with statement is not recommended, as it may be the source of confusing bugs and compatibility issues. See the "Ambiguity Contra" paragraph in the "Description" section below for details.

See why with statement is bad

"use strict";
with (Math){x = cos(2)};   // This will cause an error: Strict mode code may not include a with statement

‘arguments/eval’ as variable name

"use strict";
let arguments = 3.14;   // This will cause an error: Unexpected eval or arguments in strict mode
let eval = 3.14; // Same as above

Delete undeletable prop

"use strict";
delete Object.prototype;   // This will cause an error: Cannot delete property 'prototype' of function Object() { [native code] }

Write to get-only/read-only property

"use strict";
const obj = {get x() {return 0} };

obj.x = 3.14;   // This will cause an error

"use strict";
const obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});

obj.x = 3.14;   // This will cause an error: Cannot assign to read only property 'x' of object '#<Object>'

Other Features

Local scope

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

function myFunction() {
  "use strict";
  y = 3.14;  // This will cause an error (y is not defined).
}
</script>

Unspecified this in function

Inside functions, the “this” keyword is no longer the global object if not specified:

function myFunction() {
  alert(this);
}
myFunction(); // pops 'undefined'