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In the last video, we saw that we could remove all these different type annotations and we did not

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see any errors, in fact, if we then hovered over these variables.

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It's clear that TypeScript still understands what type of value we're going to assign to each given

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variable.

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So what you're seeing here is the type inference system in action.

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In this video, we're going to discuss how type inference works and then discuss when we're going to

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rely on type inference versus adding in an annotation manually.

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So let's get to it.

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All right.

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So first off, the basics of type inference.

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Anytime you and I make a new variable inside of our application, we're actually undergoing two separate

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steps.

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So on the left hand side of this equals sign where we say const color, we're doing something called

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variable declaration.

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This is where we are telling our program that we're going to have a new variable inside of our application

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with the given name.

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And then in the second step of this entire process, on the right hand side, the equals sign, we are

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attempting to assign a value to that variable.

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Assigning a value to a variable for the very first time is referred to as variable initialization.

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So this entire line of code right here, two separate steps, first variable declaration and then initialization.

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Here's the rule around type inference, if we do our declaration and initialization on the same line

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or essentially in one single expression, then TypeScript is going to figure out the type of value that

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we're going to assign to color for us.

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So in this case, TypeScript would look at the variable that were the value that we're usually using

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for initialization.

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It would see that this is a string and it would say, OK, color must be a variable that's going to

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refer to a value of type string.

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And that's how we get this kind of redone type right here where we can delete the annotation.

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And TypeScript still sees what's going on for us.

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If I moved this assignment or this initialization onto a separate line, then it's not going to work

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anymore.

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So if I say Apple's equals five on the next line down by hover over Apple's, you'll notice that it

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has a different annotation on here.

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It says any in this case, we're going to discuss with any means in just a moment.

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But right now, I think you can agree with me that TypeScript can no longer figure out what type of

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value we're going to assign to apples.

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So, again, the rule of thumb here is if you're doing the initialization and declaration on the same

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line, TypeScript will use type inference for us.

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OK, so that's the kind of easy part of type in France would do everything on one line, everything's

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going to work just fine.

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So now here's the really obvious question.

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When are we going to add in these type annotations?

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When are we going to type them out?

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If we're saying that TypeScript can figure out these different types for us, why would we go through

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the process of writing out the invitations manually?

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And that's a great question.

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Let's take a look at a diagram to understand why we might want to use type inference over annotation

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or the opposite.

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OK, so in that file we just put together, we've got a lot of examples of type annotations.

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So remember, that's where you and I, the developers are going to tell TypeScript the type that a value

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or a variable is going to refer to.

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Now, type inference, that's where TypeScript is going to guess everything for us.

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So in general, we're going to rely on type inference Ollis all the time, wherever we can.

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We will rely on type inference.

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That means that whenever you and I have a variable that we are declaring and then initializing on the

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same line, you and I in this course are not going to add in type annotations.

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So we're going to have a lot of code that looks like this right here.

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And we will have much more rarely have code that looks like this down here or anything else that you

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see inside this file.

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So we are going to rely upon type inference as much as we possibly can.

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But there are three scenarios where we will rely on type annotations.

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So let's take a quick break right here.

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And we're going to go through these three different examples of where we are going to add in these type

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annotations to help type script out.

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So quick pause and I'll see you in just a minute.