Starting with Javascript

• Today's sandbox
• As I have said before, my jvascript is very c/c++, but that is ok.
  • This is a tool for us this semester.
  • If you wish to master it, do some more work.
• Javascript
  • Is a client side language:
    • The program is downloaded and run on the client's computer.
    • Access to files, both upload and download is restricted.
  • Is interpreted
    • It is slow.
    • Errors may only be encountered when they are executed.
  • Javascript and HTML play together well.
  • Javascript is somewhat object oriented.
  • It has a number of built in data structures.
  • It supports dynamic typing.
    • This is sometimes a problem for c/c++ programmers.
    • As much as we hate our types, we love them too.
• As we saw last time, we can put javascript code directly in a web page.
  • This code is executed when the page is loaded.
  • We can have multiple code sections, but there is one global space.
• We will always add 'use strict' in our javascript.
  • This enables strict mode
  • It eliminates some silent errors
  • It eliminates creating variables by misspelling identifiers

  • 'use strict'
    x = 7  // is an error unless x has already been declared
           

  • It does other things as well.
• console.log
  • ctrl-shift-j in my browser.
• Declaring variables
  • var id [=value]
    • this declares a global variable.
    • use this sparingly.
  • let id [= value]
    • Declares a variable in the current block scope.
  • const id = value
    • Declares a constant.
• (some of) The types
  • number
  • string, with either ' ' or ""
  • boolean
  • undefined
  • null
  • BigInt (perhaps later)
  • Symbol (perhaps later)
  • objects
• types are not associated with identifiers.
• The typeof command will tell us the type of a variable

  • let y = 5;
    console.log(y, "is of type", typeof y);
    y  = "hello world";
    console.log(y, "is of type", typeof y);
    y = ['a','b','c']
    console.log(y, "is of type", typeof y);
    

  • produces

  • 5 "is of type" "number"
    hello world is of type string
    ["a", "b", "c"]  "is of type" "object"
    

• semicolons:
• undefined
  • The value for uninitialized variable.

  • let z
    console.log(z, typeof z)
    console.log(typeof z === "undefined")
    console.log(z === undefined);
    

  • produces

  • undefined "undefined"
    true
    true
          

• Strings
  • Strings are objects.
  • = is a deep copy.

  • let name = "bob";
    let otherName = name;
    

  • +, += are concatenation

  • otherName = otherName + "'s"
    otherName += " house";
    

  • .length provides length.

  • console.log("The length of '"+name+"' is "+name.length
    

  • [] allows read only access

  • console.log("The first letter of '"+name+"' is "+ name[0])
    

  • == is comparison with typecast
  • === is comparison without typecast

  • console.log('checking "3"== 3', "3" == 3)
    console.log('checking "3"=== 3', "3" === 3)
    

  • They have methods
    • You should use these.
• Numbers
  • +,-,/,*, %
    • Note / is not integer division.
  • The math library
    • Math.floor()
    • Math.random()
    • Math.PI
    • and others.
  • Conversions
    • parseInt()
    • parseFloat()
    • String()
• Control structures
  • for, while, do while as in c
  • if - else as in c
  • switch case but any type.
• functions

  • function FunctionName(param [=value], param[=value]...) {
        body
        return
    }
    

  • Basic types are pass by value.

  •     function StringPass(a, b="hello") {
            console.log("\tIn String Pass a = '"+a+"'");
            console.log("\tIn String Pass b = '"+b+"'");
            a = "world";
            b = "";
        }
    
    
        let str1 = "blank string";
        StringPass(str1);
        console.log("str1 = '" + str1 + "'");
    

  • Other types are are pass by reference.

  •     let list1=[1,2,3];
        StringPass(list1);
    

  • Can return anything
  • There are other ways to declare functions but we will save that for later.