JS-Stuffs

Some good-to-know stuffs about JavaScript that is not that necessary to be aware of. If someone's curious they should know these stuffs. If you already know something, feel free to contribute.

Contents

• "use strict"
• Pass by Value or Pass by Reference

"use strict"

The purpose of "use strict" is to indicate that the code should run in strict mode. In the strict mode JS gives some extra errors and warnings that is ignored in the normal mode to make debugging easy.

It is written as a string in the code. So that the browsers that doesn't support the latest version of JS can ignore it. It is usually declared in the beginning of a script or a function.

// normal mode
trump = "BAD";
console.log(trump); // BAD

// strict mode
"use strict";

trump = "BAD";
console.log(trump); // ReferenceError: x is not defined

The "use strict" directive is only recognized at the beginning of a script or a function.

Strict mode can be declared in the global or scope or in a function. When a function is in strict mode, it won't affect the codes that are outside the function;

trump = "BAD";
console.log(trump); // BAD

function makeNoakhaliBivag() {
  "use strict";
  noakhali = "BIVAG";
}

makeNoakhaliABivag(); // ReferenceError: noakhali is not defined

Stuffs not allowed in strict mode:

• Using a variable without declaring it (Check the example given before)
• Deleting a variable

  "use strict";
  
  const trump = "BAD";
  delete trump; // SyntaxError: Delete of an unqualified identifier in strict mode.
  
  function makeNoakhaliBivag() {
    const noakhali = "BIVAG";
  }
  delete makeNoakhaliABivag(); // SyntaxError: Delete of an unqualified identifier in strict mode.

• Duplicating a parameter in a function

  "use strict";
  
  function makeNoakhaliBivag(noakhali, noakhali) {
    noakhali = "BIVAG";
  } // SyntaxError: Duplicate parameter name not allowed in this context

• Octal numeric literals and octal escape characters

  "use strict";
  
  const octalNumber = 012; // SyntaxError: Octal literals are not allowed in strict mode.
  const octalNumber = "\012"; // SyntaxError: Octal escape sequences are not allowed in strict mode.

• Writing to a read-only or get-only property of an object

  "use strict";
  
  var districts = {};
  Object.defineProperty(districts, "noakhali", {
    value: "BIVAG",
    writable: false
  });
  districts.noakhali = "NOT BIVAG"; // TypeError: Cannot assign to read only property 'noakhali' of object '#<Object>'
  
  var districts = {
    get dhaka() {
      return "UNINHABITABLE";
    }
  };
  districts.dhaka = "INHABITABLE"; // TypeError: Cannot set property dhaka of #<Object> which has only a getter

• Some keyword are reserved by JS and can't be used as variable or function names like eval, arguments, let etc
• Some statements like eval(), with() etc is not allowed in the strict mode

All of the examples pointed before does work fine in normal mode!

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Pass by Value or Pass by Reference

At first, pass by value means passing the copy of an instance or only the value to a function as arguments. And pass by reference means passing the address or a reference that points to a variable.

In Pass by Value, function is called by directly passing the value of the variable as the argument. Changing the argument inside the function doesn’t affect the variable passed from outside the function. From primitive type data, JS do always Pass by Value. Here primitive data types are String, Number, Boolean, null, undefined, and symbol.

var noakhali = "ZILLA";

function makeNoakhaliBivag(noakhali) {
  noakhali = "BIVAG";
}
makeNoakhaliBivag(noakhali);

console.log(noakhali); // ZILLA

In Pass by Reference, function is called by directly passing the reference/address of the variable as the argument. Changing the argument inside the function affect the variable passed from outside the function. In JS objects and arrays are passed by reference to a function.

var ObaidulKader = { personality: "XOSS" };

function makeObaidulKaderCool(ObaidulKader) {
  ObaidulKader.personality = "COOL";
}
makeObaidulKaderCool(ObaidulKader);

console.log(ObaidulKader.personality); // COOL

But if a new object or array is assigned to the passed variable in the Pass by Reference, then the variable passed from outside remains unchanged. Because, the reference passed to the function gets changed if we create assign a new array or object to the variable.

var ObaidulKader = { personality: "XOSS" };

function makeObaidulKaderCool(ObaidulKader) {
  ObaidulKader = { personality: "COOL" };
}
makeObaidulKaderCool(ObaidulKader);

console.log(ObaidulKader.personality); // XOSS

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