JS terms

jsTerms.md

• Call-site: The location in code where a function is called (not where it's declared). Usually the call-site we care about is in the invocation before the currently executing function

• Call-stack: The stack of functions that have been called to get us to the current moment in execution

• Closure:

  • When a function still has reference to its outer scope (parameters, variables, functions) even after that outer function has been executed.

  • Closure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope E.g

  function foo() {
      var a = 2;
  
      function bar() {
          console.log(a);
      }
  
      return bar;
  }
  
  var baz = foo();
  
  baz(); // => 2

  • https://github.com/getify/You-Dont-Know-JS/blob/master/scope%20%26%20closures/ch5.md

• Execution Context (Activation Record):

  • When a function is invoked, an activation record, otherwise known as an execution context, is created.

  • This record contains information about where the function was called from (the call-stack), how the function was invoked, what parameters were passed etc.

  • One of the properties of this record is the this reference which will be used for the duration of that function's execution.

• Lexical Scope:

  • The value of an object is determined by where it was declared not where it was called from. E.g

  function foo() {
      console.log(a); // => 2
  }
  
  function bar() {
      var a = 3;
      foo();
  }
  
  var a = 2;
  
  bar(); // => 2

• TDZ (Temporal Dead Zone):

  • Both let and const are hoisted within the lexical scope in which they are defined.

  • They are also declared and initialized to undefined similar to var but the initialization step happens at a later stage.

  • The initialization is not done at the entering of the scope (as with var) but rather happens once the compiler gets to the let statement. Eg let foo

  • If you try to access the let or const variable before this initialization step the browser will throw a ReferenceError

  • This is called a TDZ error; it basically means that you're trying to access a variable before it has been initialized to undefined

  • https://bitsofco.de/variable-and-function-hoisting-in-es2015/#post-2936836202 - Kyle Simpson comment