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So we're using the core types of no boolean and string.

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And here in the list of parameters of dysfunction we're always explicitly assigning to types with a

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colon.

4
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Often the parameter name and then the name of the type.

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So here the names are no boolean and string.

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And in case you were wondering these are now special identifiers.

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This overall here is a special syntax which is added by typescript.

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It's not part of the compile javascript code.

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If we check that code here these type assignments are gone because javascript does not support them.

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Does colon thing after a variable or after a parameter and then number and boolean and string these

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special keywords.

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This is introduced by typescript the typescript compiler understands it.

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The idea e here supports TypeScript and therefore doesn't complain about these special keywords.

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Javascript does not understand the syntax it doesn't understand a colon after a parameter where a colon

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after a variable and then the special number or string keyword javascript doesn't understand this and

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therefore does is not part of the javascript output.

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It's really just use but it types with compiler and then well it is a compiler because it converts this

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code to JavaScript code.

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Side note you also see it switches concept for Wah but that's something I'll come back to later.

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So we have our explicit type assignments here and it's only understood by typescript right.

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Why don't we have them down there.

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I don't have explicit type of assignments here and by the way also not here when we calculate the result

23
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for example because typescript has a built in feature which is called Type inference This means that

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typescript does its best and it does a pretty good job there to understand which type you have in a

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certain variable or a constant.

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And here for example it understands that no one will always be of type no in the end because you initialize

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it with a number.

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Now this actually is a constant.

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So it's even more specific than that.

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And the type it identifies here is not just any number it's the number five because you'll not be able

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to assign a new number or a new value to a concept value anyways.

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If you what changed is to a variable.

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So if you would use let instead then of course this would not break anything we can use a variable there.

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It might be best practice because there's value never changes but it's also not horrible.

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But now if we hover over this we see typescript doesn't say okay.

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This has to be a 5.

37
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But still it detects that the type here is a number.

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Now we absolutely could write this code here on our own.

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We can add a colon after the variable name on the left side of the equal sign and then the name of the

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type.

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So basically the same we did in the parameters of this function but does this redundant and it actually

42
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is also not considered to be a good practice because typescript is able to perfectly infer this type

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from there.

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So assigning this is not a good idea.

45
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This only changes if you would create this variable in a unassigned way.

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Like this if you don't initialize it immediately then it's a good practice to tell typescript which

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value will eventually be stored in there so that when you later assign a value to it.

48
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And of course it's a bit redundant here to split this in two lines.

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I'm just doing this for demo purposes but now if I assigned this here this works because I told typescript

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in advance that this will be of type No you don't have to do that.

51
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It also works if you don't do that but now you could also add this 5 and you wouldn't get an error because

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you're not telling typescript anything about the type which will be stored in this variable and therefore

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typescript allows any type.

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If you instead add code a number here you're telling typescript hey eventually a number will be stored

55
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in there and hence if you later stored something else and there like in this case where we store a string

56
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you'll get an error here in the idea.

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And of course also if you compile your code we'll get the error.

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We already saw before so this is how you can assign types.

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Now even if types just inferred a type and let me fix this it will yell at you if you break that inferred

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type.

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Actually it's wrong to say even because of course it does.

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Why would it not do that type and for instance dare for you to save you code to a why did you manually

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have to assign a type of course typescript yells at you if you then use a wrong type a type that it

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did not infer.

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For example here if resolve phrase is created with left so that it's variable then type code here infers

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that this will be of type String because we initialize it with a string.

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So this is basically the equivalent to not initializing it and setting the type on our own and then

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assigning a value later.

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Now if we change result phrase to let's say 0 here for whatever reason we might want to do that well

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then we get an error here.

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That type 0 is not a signal of type String and that makes sense right typescript inferred that we want

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to store a string.

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We're now trying to store a number we get an error.

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That's the job.

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The core task of typescript checking typos and yelling at us if we're using them incorrectly.