#100DaysOfWeb in Python Transcripts
Chapter: Days 25-28: JavaScript Introduction
Lecture: Variables (var vs let) and data types
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In this video, we're going to look at JavaScript basics. Let's open a console and write some JavaScript.
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So the first way to do a Hello World to the console is to use console.log. Second way, we can do it in the browser is to use the built in alert.
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And that gives us a nice pop up. To define a variable you can use var. But var is a bit outdated and these days it's best practice to use let.
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This creates it's own scope and we'll see that in a bit. Let's rename it something else.
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The advantage is that I can not redefine my variable. Where var was more permissive. JavaScript has a reserved word to define constant and it's const.
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I'm using camel case as is best practiced in JavaScript supposed to hyphen which would use underscores.
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Okay, for example set an min age for driving at 18 and now I can not set this to another value. Because it's defined as a constant.
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So just play a string. Use alert or console.log. And especially when you start to write your JavaScript code print it to the console is one way
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to debug your programs. But I will show you more tricks in a later video. Define variables, already JavaScript used var
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but now best practice is to use let and constants are defined with const. And we saw that one advantage of let is that you can not redefine a variable
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and a constant can not be assigned another value. Var vs let. Let was introduced with ECMAScript 2015 let variables are Blocked Scope.
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As we know from the Zen of Python name spaces are a good idea and that's exactly what's going on here because if you give a variable scope
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then it's more difficult that variables take on values that you wouldn't expect. And here we see a practical example where with the old style, var
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the variable will still be available outside of the block. And block I define as whatever goes into curvy braces. However with let the variables scoped
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inside of this block and if you try to reference it outside of that block you get a reference error and that's way safer you can read more about that
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on Mozilla's developer documentation. I want to introduce this early on in the lesson because it's often that you define a variable
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so it's good to know best practice. JavaScript has primitives and objects. Primitives are boolean, null undefined, number string and symbol.
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It works pretty much as you would expect. We're all familiar with boolean from Python the only thing that like cause confusion
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is null and undefined, where Python generally has None. The more options you have the more likely it's that you'll make a mistake.
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Another tricky thing is one type for numbers which is number. And Python has ins and close which usually leads to less mistakes.
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String you already know from Python str. And symbol is a new data type I have not found a use case yet so it's the youngest cub of this chapter.
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Objects are very important in JavaScript at the most basic level you can use them as ticks of key value mappings
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and I will have an example latter in this lesson. But there's much more to it you can actually use them to add methods and behavior.
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Making them what in Python would be fully flat classes. More info on data types here's the link. Be warned that JavaScript is extremely permissive
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when you add different types together and that puts the responsibility on developer. A nice example is summing an int and a string.
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In Python it will lead to a type error which probably is a good thing because Python doesn't really understand what you want to do.
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However JavaScript makes that decision implicitly for you in this case it adds it to as if there were strings.
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So in this case you need to do a parseInt on the 37th string to get an integer result. So just keep that in mind that often times where Python crashes
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Java Script does something implicit and it might not be what you want. Towards the end of this lesson I have a whole slide on JavaScript contrast
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and it's important to become familiar with how the language is designed. In the next video we're going to look at JavaScript erase.