book2-chap1-quiz.answers.md

Quiz - YDKJS: Scope & Closures 1/5

Chapter 1. What is Scope?

Answer Sheet

Section: Compiler Theory

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1. What are the three typical steps in a traditional compiled-language process?

1. Tokenizing / Lexing

2. Parsing

3. Code-Generation

2. What does AST stands for?

Abstract Syntax Tree

3. So, is JS interpreted or compiled? Explain why

JS is compiled as compilation happens just before (microseconds) the code is executed.

Section: Understanding Scope

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4. Once again: What's Scope?

a well-defined set of rules for storing and finding variables (identifiers) in some location.

5. What are the main "characters" involved in the compilation process?

engine

compiler

scope

6. Describe both the LHS and RHS look-up terms.

LHS Left-hand Side look-up. Process of looking for a variable (target) to assign it a value.

RHS Right-hand Side look-up. Process of retrieving the value of a variable (source).

7. Identify all the LHS and RHS look-ups for the following code snippet:

Snippet #1

function square(number) {
  return number * number;
}

function cube(number) {
  return number * square(number);
}

const result = cube(3);

total LHS look-ups: 3

• assignment to result = ...

• implicit param assignment number = 3 in fuction cube(number)

• implicit param assignment number = 3 in fuction square(number)

total RHS look-ups: 6

• cube(3) function execution

• inside cube function > retrieve the value for number in number *

• inside cube function > execution of square(...)

• inside cube function > retrieve the value of argument number in square(number)

• inside square function > retrieve value for number in number *

• inside square function > retrieve value for number in * number

Section: Nested Scope

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8. What are the rules for traversing a nested Scope?

engine starts at the currently executing Scope, then looks for a variable there, if not found, keeps going up one level an so on util it reaches the outermost global scope and stops whether it finds the variable or not.

9. What would be the output for the following snippets?

Snippet #2

g = 10;

function foo(a) {
  function bar(a, b) {
    var c = 5;

    function baz(d) {
      return d + c + b + a + g;
    }

    return baz(5);
  }

  return bar(10, a + 5);
}

var g;

console.log(foo(15));

50

Snippet #3

(function foo(x) {
  function bar(y) {
    function baz(z) {
      w = x * y * z;

      return x + y + z;
    }

    return baz(4);
  }

  return bar(4);
})(4);

console.log(w);

64

Section: Errors

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10. When does a ReferenceError happen?

when a variable couldn't be found in any of the Scopes.

11. When does a TypeError happen?

when a variable is found but an illegal/impossible action is attempted against its result.

12. Write the output for the following snippets:

Snippet #4

function bar(n) {
  return n * m;
  var m = 2;
}

console.log(bar(3));

NaN

Snippet #5

function bar(n) {
  return n * m;
  m = 2;
}

console.log(bar(3));

ReferenceError: m is not defined

Snippet #6

function bar(n) {
  m = 2;
  return n * m;
}

console.log(bar(3));

6

Snippet #7

'use strict';

function bar(n) {
  m = 2;
  return n * m;
}

console.log(bar(3));

ReferenceError: m is not defined

Snippet #8

function bar(n) {
  var m = 2;
  return n * m;
}

bar = 5;

console.log(bar(3));

TypeError: bar is not a function

Snippet #9

var bar;

function bar(n) {
  var m = 2;
  return n * m;
}

console.log(bar(3));

6

Snippet #10

var bar = 3;

function bar(n) {
  var m = 2;
  return n * m;
}

console.log(bar(3));

TypeError: bar is not a function

Snippet #11

function bar(n) {
  var m = 2;
  return n * m;
}

console.log(bar(3));

var bar = 3;

6

Snippet #12

var bar = 2;
var bar = 'a';

console.log(bar);

"a"