ES6: Compare Scopes of the var and let Keywords

When you declare a variable with the var keyword, it is declared globally, or locally if declared inside a function.

The let keyword behaves similarly, but with some extra features. When you declare a variable with the let keyword inside a block, statement, or expression, its scope is limited to that block, statement, or expression.

For example:

var numArray = [];

for (var i = 0; i < 3; i++) {

  numArray.push(i);

}

console.log(numArray);

// returns [0, 1, 2]

console.log(i);

// returns 3

With the var keyword, i is declared globally. So when i++ is executed, it updates the global variable. This code is similar to the following:

var numArray = [];

var i;

for (i = 0; i < 3; i++) {

  numArray.push(i);

}

console.log(numArray);

// returns [0, 1, 2]

console.log(i);

// returns 3

This behavior will cause problems if you were to create a function and store it for later use inside a for loop that uses the i variable. This is because the stored function will always refer to the value of the updated global i variable.

var printNumTwo;

for (var i = 0; i < 3; i++) {

  if(i === 2){

    printNumTwo = function() {

      return i;

    };

  }

}

console.log(printNumTwo());

// returns 3

As you can see, printNumTwo() prints 3 and not 2. This is because the value assigned to i was updated and the printNumTwo() returns the global i and not the value i had when the function was created in the for loop. The let keyword does not follow this behavior:

'use strict';

let printNumTwo;

for (let i = 0; i < 3; i++) {

  if (i === 2) {

    printNumTwo = function() {

      return i;

    };

  }

}

console.log(printNumTwo());

// returns 2

console.log(i);

// returns "i is not defined"

i is not defined because it was not declared in the global scope. It is only declared within the for loop statement. printNumTwo() returned the correct value because three different i variables with unique values (0, 1, and 2) were created by the let keyword within the loop statement.

Fix the code so that i declared in the if statement is a separate variable than i declared in the first line of the function. Be certain not to use the var keyword anywhere in your code.

This exercise is designed to illustrate the difference between how var and let keywords assign scope to the declared variable. When programming a function similar to the one used in this exercise, it is often better to use different variable names to avoid confusion.

Basic JavaScript: Profile Lookup

We have an array of objects representing different people in our contacts lists.

A lookUpProfile function that takes name and a property (prop) as arguments has been pre-written for you.

The function should check if name is an actual contact's firstName and the given property (prop) is a property of that contact.

If both are true, then return the "value" of that property.

If name does not correspond to any contacts then return "No such contact"

If prop does not correspond to any valid properties of a contact found to match name then return "No such property"

Iterate with JavaScript For Loops

You can run the same code multiple times by using a loop.

The most common type of JavaScript loop is called a "for loop" because it runs "for" a specific number of times.

For loops are declared with three optional expressions separated by semicolons:

for ([initialization]; [condition]; [final-expression])

The initialization statement is executed one time only before the loop starts. It is typically used to define and setup your loop variable.

The condition statement is evaluated at the beginning of every loop iteration and will continue as long as it evaluates to true. When condition is false at the start of the iteration, the loop will stop executing. This means if condition starts as false, your loop will never execute.

The final-expression is executed at the end of each loop iteration, prior to the next condition check and is usually used to increment or decrement your loop counter.

In the following example we initialize with i = 0 and iterate while our condition i < 5 is true. We'll increment i by 1 in each loop iteration with i++ as our final-expression.

var ourArray = [];
for (var i = 0; i < 5; i++) {
  ourArray.push(i);
}
ourArray will now contain [0,1,2,3,4].


Use a for loop to work to push the values 1 through 5 onto myArray.

Basic JavaScript: Record Collection

You are given a JSON object representing a part of your musical album collection. Each album has several properties and a unique id number as its key. Not all albums have complete information.

Write a function which takes an album's id (like 2548), a property prop (like "artist" or "tracks"), and a value (like "Addicted to Love") to modify the data in this collection.

If prop isn't "tracks" and value isn't empty (""), update or set the value for that record album's property.

Your function must always return the entire collection object.

There are several rules for handling incomplete data:

If prop is "tracks" but the album doesn't have a "tracks" property, create an empty array before adding the new value to the album's corresponding property.

If prop is "tracks" and value isn't empty (""), push the value onto the end of the album's existing tracks array.

If value is empty (""), delete the given prop property from the album.

Hints
Use bracket notation when accessing object properties with variables.

Push is an array method you can read about on Mozilla Developer Network.

You may refer back to Manipulating Complex Objects Introducing JavaScript Object Notation (JSON) for a refresher.

Accessing Nested Arrays

As we have seen in earlier examples, objects can contain both nested objects and nested arrays. Similar to accessing nested objects, Array bracket notation can be chained to access nested arrays.

Here is an example of how to access a nested array:

var ourPets = [
  {
    animalType: "cat",
    names: [
      "Meowzer",
      "Fluffy",
      "Kit-Cat"
    ]
  },
  {
    animalType: "dog",
    names: [
      "Spot",
      "Bowser",
      "Frankie"
    ]
  }
];
ourPets[0].names[1]; // "Fluffy"
ourPets[1].names[0]; // "Spot"

Retrieve the second tree from the variable myPlants using object dot and array bracket notation.

Basic JavaScript: Using Objects for Lookups

Objects can be thought of as a key/value storage, like a dictionary. If you have tabular data, you can use an object to "lookup" values rather than a switch statement or an if/else chain. This is most useful when you know that your input data is limited to a certain range.

Here is an example of a simple reverse alphabet lookup:

var alpha = {
  1:"Z",
  2:"Y",
  3:"X",
  4:"W",
  ...
  24:"C",
  25:"B",
  26:"A"
};
alpha[2]; // "Y"
alpha[24]; // "C"

var value = 2;
alpha[value]; // "Y"

Convert the switch statement into an object called lookup. Use it to look up val and assign the associated string to the result variable.

Basic JavaScript: Local Scope and Functions

Declare a local variable myVar inside myLocalScope. Run the tests and then follow the instructions commented out in the editor.

Hint

Refreshing the page may help if you get stuck.