Code 301: Pre-work Readings

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Can I use: Arrow Functions

Read: # MDN Arrow Functions

• Compact alternative to a traditional function expression
• No binding to this or super & should not be used as a method
• No arguments or new.target keywords
• Cannot be used as a constructor
• Example:
  • Traditional

  
  function (a) {
    return a + 100;
  }
  
  

  • Arrow Function

  
  a => a + 100;
  
  

• Traditional 2

  
  function (a, b) {
    return a + b + 100;
  }
  
  

  • Arrow Function 2

  
  (a, b) => a + b + 100;
  
  

• Traditional 3

  
  let a = 4;
  let b = 2;
  function (a) {
    return a + b + 100;
  }
  
  

  • Arrow Function 3

  
  let a = 4;
  let b = 2;
  a => a + b + 100;
  
  

• Traditional 4

  
  function (a, b) {
    let chuck = 42;
    return a + b + chuck;
  }
  
  

• Arrow Function 4

  
  (a, b) => {
    let chuck = 42;
    return a + b + chuck;
  }
  
  

• Traditional 5

  
  function bob (a) {
    return a + 100;
  }
  
  

• Arrow Function 5

  
  let bob = a => a + 100;
  
  

• Basic syntax: parameter => expression
  • (parameter1, parameter2) => expression

Read: JavaScript Arrow Functions ES6 - Wes Bos

• Arrow function benefits: concise syntax, implicit returns, do not rebind value of this
• Operand => called a “fat arrow”
• Arrow functions are all anonymous and do not have names
• Can assign a variable the value of an arrow function

Read: Let and Const

Let MDN

Can I use: let

• let declares variables that are limited to the scope of a block statement, or expression
  • Only initialized to a value when a parser evaluates it
  • Does not create properties of the window object when declared globally
    • Example:

    
        var x = 'global';
        let y = 'global';
        console.log(this.x); // "global"
        console.log(this.y); // undefined
    
    

Const MDN

Can I use: let

• Constants block scoped, can’t be changed through reassignment and can’t be redeclared
• Global constants do not be come properties of the window object
• Must assign value at time of declaration
• const declaration creates a read-only reference to a variable
• Cannot share a name with a a function or a variable in the same scope
• Can be declared with upper or lower case, convention is to use all-uppercase letters

Video: Var, Let and Const

• Constants minimize mutable state!

Read: JavaScript ES6+: var, let, or const?

• typeof does not work with let or const

Read: let vs const ES6 - Wes Bos

• Whenever you are inside of a set of curly braces you are in block scope
• const cannot be assigned a new value after declaration, but if it is an object its properties can be updated

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