Let.md

Let

• let is visible only block scope.
• var can re-assign the same variable in the same scope.
• let can NOT re-assign the same variable in the same scope.
• Here is the block scope.

{
   // this is a block 
}

Function scope

Q1

function hello(){
  var a = "function";
}
hello();
console.log(a);

Q1-Anwer

//Reference error
//var a is only visible in function scope

---

Q2

function hello(){
  var a = "function";
  
  for(var i=0; i<10; i++){
    var a = "block";
  }
  console.log(a)
}
hello();

Q2-Answer

// block
// var a='block' in for() loop is visible outside scope.

---

Q3

• function scope with IIFE

function hello(){
  var a = "function";
  
  for(var i=0; i<10; i++){
    (function(){ // Inside IIFE isn't visible from outside of this function.
      var a = "block";
    })();
  }
  console.log(a)
}
hello();

Q3-Answer

// function
// Inside IIFE isn't visible from outside 

---

Q4

• let in block scope

function hello(){
  var a = "function";
  
  for(var i=0; i<10; i++){
    let a = "block"; // this line is only visible in for loop block. That's why a prints "function".
  }
  console.log(a);
}
hello(); 

Q4-Answer

// function

Quiz

Quiz1

function hello(){
  var a = "function";
  
  for(let i=0; i<10; i++){
    a = "block";
  }
  console.log(a);
}
hello(); 

Quiz1-Anwer

// block
// "a" is defined with var so variable a in for() loop is not block scope.

---

Quiz2

var funcs = [];
for (var i = 0; i < 5; i += 1) {
  funcs.push(function () {
    console.log(i);
  })
}
funcs.forEach(function (func) {
  func()
});

Quiz2-Answer

var funcs = [];
for (var i = 0; i < 5; i += 1) { // var i is a global variable 
  funcs.push(function () {
    console.log(i); // added function as funcs array
  })
}
//Here...., i=5
funcs.forEach(function (func) {
  func() // When func is executed, i is always 5!
});
// 5 times print out 5

---

Quiz3

• ES5 block scope question

var funcs = [];

for (var i = 0; i < 10; i += 1) {
  var y = i;
    funcs.push(function () {
    console.log(y);
  })
}
funcs.forEach(function (func) {
  func()
});

Quiz3-Answer

var funcs = [];
//var y; 
for (var i = 0; i < 10; i += 1) {
  var y = i; // y in for() loop is global scope NOT block scope.
    funcs.push(function () {
    console.log(y);
  })
}
// Here... y=9
funcs.forEach(function (func) {
  func() //// 10 times print out "9"
});

---

Quiz4

var funcs = [];
for (var i = 0; i < 10; i += 1) {
  let y = i;
    funcs.push(function () {
    console.log(y);
  })
}
funcs.forEach(function (func) {
  func()
});

Quiz4-Answer

var funcs = [];
for (var i = 0; i < 10; i += 1) {
  let y = i;
    funcs.push(function () {
    console.log(y);
  })
}
//Here.... let y=i is NOT visible. 
funcs.forEach(function (func) {
  func()
});
// 0
// 1
// 2
// 3
// 4
// 5
// 6
// 7
// 8
// 9

---

Quiz5

var funcs = [];
for (let i = 0; i < 10; i += 1) {
    funcs.push(function () {
    console.log(i);
  })
}
funcs.forEach(function (func) {
  func()
});

Quiz5-Answer

var funcs = [];
for (let i = 0; i < 10; i += 1) {
    funcs.push(function () {
    console.log(i);
  })
}
//Here... let i is NOT visible because let i is block scope.
funcs.forEach(function (func) {
  func()
});
// 0
// 1
// 2
// 3
// 4
// 5
// 6
// 7
// 8
// 9

Redeclarations

Redeclaring the same variable within the same function or block scope raises a SyntaxError. MDN let- Redeclarations

Quiz6-1

let nameLet = 'Jen';
let nameLet = 'Julie';
console.log('nameLet', nameLet);

Quiz6-1-Answer

//Uncaught SyntaxError: Identifier 'nameLet' has already been declared
console.log('nameLet', nameLet);

---

Quiz6-2

if (x) {
  let foo;
  let foo; 
}

Quiz6-2-Answer

if (x) {
  let foo;
  let foo; // SyntaxError thrown.
}

---

Quiz7

let x = 1;
switch(x) {
  case 0:
    let foo;
    break;
    
  case 1:
    let foo;
    break;
}

Quiz7-Answer-1

let x = 1;
switch(x) {
  case 0:
    let foo;
    break;
    
  case 1:
    let foo; // SyntaxError for redeclaration.
    break;
}

Quiz7-Answer-2

• Adding {} in case works fine.

let x = 1;

switch(x) {
  case 0: {
    let foo;
    break;
  }  
  case 1: {
    let foo;
    break;
  }
}