Previously, we covered various approaches that you can take to narrowing types in Typescript. Type narrowing is the process of moving the type of a variable from a less precise to a more precise type i.e. from a union of string and number to string only. You can learn more about type narrowing here.

In this article, we are going to look at how we can create our own custom type guards. Custom type guards will help you to check if a variable is of a certain type before usage, which helps in Type narrowing.

Take for instance the following function, which calculates the area of a shape i.e. Circle or Rectangle.

```
function calculateArea(shape: Rectangle | Circle) {
// calculate area
}
```

In order to calculate the area, we will need to determine whether the shape being passed in is a Rectangle or Circle. We can create a custom type guide that will check if the type of a `Rectangle`

and calculate its area, otherwise calculates the area of a circle:

```
if(isRectangle(shape)) {
// calculate area of a rectangle
} else {
// calculate area of a circle
}
```

## What is a Type Predicate?

A type predicate is a function return type that tells typescript a parameter is of a specific type. A predicate takes the following format: `parameterName is Type`

, where `parameterName`

must be the name of a parameter in the function parameter signature.

For instance, if we wanted to build the custom type guard `isRectangle`

above, our type predicate would be `shape is Rectangle`

, where `shape`

is the parameter name, as shown below.

```
function isRectangle(shape: unknown): shape is Rectangle {
// function body
}
```

## Custom Type Guard

To define a custom type guard, we create a function that returns a type predicate. The function itself just needs to return true or false. If we take the example above for `isRectangle`

type guard, we would check if the `width`

and the `height`

are present and return `true`

, otherwise, return `false`

.

```
function isRectangle(shape: unknown): shape is Rectangle {
if ("width" in shape && "height" in shape) {
// this is a rectangle
return true;
}
// it's not a rectangle
return false;
}
```

In the above example, we are using Javascripts in operator to check if the width and height properties are in the shape object.

### Usage

To use the custom type guard, we use it just like any other function that returns a boolean.

```
type Rectangle = {
height: number;
width: number;
}
type Circle = {
radius: number;
}
const r: Rectangle = {
height: 12,
width: 15
}
const c: Circle = {
radius: 10,
}
console.log(isReactangle(r)); // true
console.log(isReactangle(c)) // false
```

By using it within a control flow, you can narrow the type of the variable, just like other methods of narrowing types.

```
function area(shape: Rectangle | Circle) {
if(isRectangle(shape)) {
// Rectangle
shape.height // no error
shape.radius // error
} else {
// Circle
shape.radius // no error
shape.height // error
}
}
```

## Conclusion

In this brief article, we learned what a Type predicate is and how to build custom type guards. We learned that a type guard is a special function that returns a type predicate so that typescript is able to determine the type of a variable.

We will continue covering similar topics in Typescript in this series - A Byte of Typescript. A Byte of Typescript is a new series that I will be publishing on a regular basis to help you demystify Typescript.

If you are looking to learn more about Typescript, here are the previous articles I have published. Thank you ðŸ˜„.

## Top comments (7)

Do you know what the difference between

`shape is Rectangle`

and`asserts shape is Rectangle`

is?I don't understand, how is the second one used?

I

thinkI understand it. If a return type is described as`x is y`

, then typescript assumes that if the function returns true`x`

is`y`

.However, with

`asserts x is y`

, typescript will assume that the function will throw if`x`

isn't`y`

.Super cool!

Yeah, that is true, it's upto you to do an exhaustive check on whether an object is of that type before returning true.

Yeah, I understood the concept, I was just confused by the difference of

`asserts x is y`

vs`x is y`

Yeah, that's true. I probably should add that you need to do an exhaustive check on the type before returning it.

Yeah, it can but I find this shorthands to be unreadable especially for the context of a tuturial, So I chose to be as verbose as possible.