description: Review

Review-Of-Previous-Concepts

Review of Concepts

Running JS Locally Concepts

• Match the commands ls, cd, pwd to their descriptions
  • ls lists contents of current directory
  • cd changes current directory
    • cd .. takes you up one level
    • cd alone takes you back home
  • pwd returns current directory
• Given a folder structure diagram, a list of ‘cd (path)’ commands and target files, match the paths to the target files.
• Use VSCode to create a folder. Within the folder create a .js file containing console.log('hello new world'); and save it.
• Use node to execute a JavaScript file in the terminal

Plain Old JS Object Lesson Concepts

• Label variables as either Primitive vs. Reference

  • primitives: strings, booleans, numbers, null and undefined
    • primitives are immutable
  • refereces: objects (including arrays)
    • references are mutable

• Identify when to use . vs [] when accessing values of an object

  • dot syntax object.key
    • easier to read
    • easier to write
    • cannot use variables as keys
    • keys cannot begin with a number
  • bracket notation object["key]
    • allows variables as keys
    • strings that start with numbers can be use as keys

• Write an object literal with a variable key using interpolation

  • put it in brackets to access the value of the variable, rather than just make the value that string

    let a = 'b';let obj = { a: 'letter_a', [a]: 'letter b' };
    

• Use the obj[key] !== undefined pattern to check if a given variable that contains a key exists in an object

  • can also use (key in object) syntax interchangeably (returns a boolean)

• Utilize Object.keys and Object.values in a function

  • Object.keys(obj) returns an array of all the keys in obj
  • Object.values(obj) returns an array of the values in obj

• Iterate through an object using a for in loop

  let printValues = function (obj) {    for (let key in obj) {        let value = obj[key];        console.log(value);    }};
  

• Define a function that utilizes ...rest syntax to accept an arbitrary number of arguments

  • ...rest syntax will store all additional arguments in an array

  • array will be empty if there are no additional arguments

    let myFunction = function (str, ...strs) {    console.log('The first string is ' + str);    console.log('The rest of the strings are:');    strs.forEach(function (str) {        console.log(str);    });};
    

• Use ...spread syntax for Object literals and Array literals

  let arr1 = ['a', 'b', 'c'];let longer = [...arr1, 'd', 'e']; // ["a", "b", "c", "d", "e"]// without spread syntax, this would give you a nested arraylet withoutRest = [arr1, 'd', 'e']; // [["a", "b", "c"], "d", "e"]
  

• Destructure an array to reference specific elements

  
  let array = [35,9];
  
  let [firstEl, secondEl] = array;
  
  console.log(firstEl); // => 35
  
  console.log(secondEl); // => 9