C. JS exclusive features #

Created Thursday 25 March 2021

1. Symbol data type #

• Useful when working with objects.

• Used to create properties in objects which are invisible to the object owner.

• This is important if different people work on the same object. Separation of concerns is the goal, symbols avoid properties to clash, if the object is handled by two scripts.

• There is a global symbol registry, syntax, Symbol.for(‘id’) is used to get the key for id, this is useful if we want some symbols to be the same in a team where a project has many teams.

• Symbol() don’t auto-convert to strings, which is why obj[id1] is different for different people.

  id = Symbol('id'); // desciption does not affect the hash value
  const obj = {
  	[id]: 23;
  }
  
  // we can add our own properties to an existing object
  id1 = Symbol();
  obj[id1] = 231; // not accessible to the original user even if he uses property name as id1
  

  FIXME

2. use strict #

• Purpose - Modern mode for JS, generates more exceptions that usual. It can be enforced at the function or script level. See.

• Syntax - add "use strict" at the first line inside the function, class or script. Only comments may appear about the "use script".

• It is just a signal for the interpreter.

• Gotcha - There’s no way to “cancel” "use strict", once enabled.

• Example

  // Example 1 - Globally
  "use strict"; // for the whole script
  
  ///_ code_/
  
  // Example 2 - Selective
  function f()
  {
  	"use strict"; // for the function
  	// code
  }
  function g()
  {
  	/_code_/ // no effect of "strict"
  }
  

• Is it used often? There are two cases:

  • Code using classes and modules - Not required. "use strict" is enabled inside these by default.
  • Old codebases - "use strict" is encouraged.