Why are Actions Not Allowed in Strict Mode?

Some typical actions that are forbidden in strict mode are given below:

1. Undeclared objects or variables.

We have already seen that using a variable or an object without declaring it is not allowed in strict mode.

Example:

"use strict";
user = {
    name: 'John',
    age: 30,
    gender: 'male',
}
 
console.log(user.name) // John

Output:

user = {
referenceError: User is not defined

2. Deleting an object, variable, or function.

Example:

"use strict";
let user = {
    name: 'John',
    age: 30,
    gender: 'male',
}
 
delete user; // Error here

Output:

delete user = {
SyntaxError: Delete of …..

3. Duplicating a parameter name.

Example:

"use strict"; functionhello(value, value){     console.log(value + " "+ value);}

Output:

function hello(value, name){

user.name = “John”;

4. Writing to a read-only property, get-only property & undeletable property is not allowed.

Example:

"use strict";
const user = {};
Object.defineProperty(user, "name", {value:0, writable:false});
 
user.name = "John";

Output:

user.name = “John”;

TypeError: Cannot …

4. Octal numeric literals & Octal escape characters are not allowed.

Example:

"use strict";
letnum = "\010";  
// Error here

Output:

let num = "\010";   // Error here

SyntaxError:…

5. ‘eval’ cannot be used as a variable.

Example:

"use strict";
let eval = "Hello World";
 
console.log(eval);

Output:

let eval = "Hello World";

SyntaxError:…

6. ‘arguments’ cannot be used as a variable.

Example:

"use strict";
let arguments = "Hello World";
 
console.log(arguments);

Output:

let arguments = “Hello World”;

SyntaxError:…

7. ‘with’ is not allowed.

Example:

"use strict";
with ([1, 2, 3]) {
    console.log(toString()); 
}

Output:

with ([1, 2, 3]) {

SyntaxError:…

8. Keywords such as implements, interface, static, package, private, protected, public, let and yield are prohibited in strict mode.

Example:

"use strict";
let public = "public"; 

Output:

let public = “public”;

SyntaxError:…