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TypeScript Type Inference Explained By Practical Examples

Summary: in this tutorial, you will learn about type inference in TypeScript.

Type inference describes where and how TypeScript infers types when you don’t explicitly annotate them.

When you declare a variable, you can use a type annotation to explicitly specify a type for it. For example:

let counter: number;Code language: JavaScript (javascript)

However, if you initialize the counter variable with a number, TypeScript will infer the type of the counter to be number. For example:

let counter = 0;Code language: JavaScript (javascript)

It is equivalent to the following statement:

let counter: number = 0;Code language: JavaScript (javascript)

Likewise, when you assign a function parameter a value, TypeScript infers the type of the parameter to the type of the default value. For example:

function setCounter(max=100) {
    
}Code language: JavaScript (javascript)

In this example, TypeScript infers the type of the max parameter to be number.

Similarly, TypeScript infers the following return type of the increment() function as number:

function increment(counter: number) {
    return counter++;
}Code language: JavaScript (javascript)

It is the same as:

function increment(counter: number) : number {
    return counter++;
}Code language: JavaScript (javascript)

Consider the following assignment:

let items = [1, 2, 3, null];Code language: JavaScript (javascript)

To infer the type of items variable, TypeScript needs to consider the type of each element in the array.

It uses the best common type algorithm to analyze each candidate type and select the type that is compatible with all other candidates.

In this case, TypeScript selects the number or null array type (number | null) []) as the best common type. Note that the | means the OR operator in types.

If you add a string to the items array, TypeScript will infer the type for the items as an array of numbers and strings: (number | string)[]

let items = [1, 2, 3, 'Cheese'];Code language: JavaScript (javascript)

TypeScript uses the locations of variables to infer their types. This mechanism is known as contextual typing. For example:

document.addEventListener('click', function (event) {
    console.log(event.button); 
});Code language: JavaScript (javascript)

In this example, TypeScript knows that the event parameter is an instance of MouseEvent because of the click event.

However, when you change the click event to the scroll the event, TypeScript will issue an error:

document.addEventListener('scroll', function (event) {
    console.log(event.button); 
});Code language: JavaScript (javascript)

Error:

Property 'button' does not exist on type 'Event'.(2339)Code language: JavaScript (javascript)

TypeScript knows that the event in this case, is an instance of UIEvent, not a MouseEvent. And UIEvent does not have the button property, therefore, TypeScript throws an error.

You will find contextual typing in many cases such as arguments to function calls, type assertions, members of objects and array literals, return statements, and right-hand sides of assignments.

The following shows the difference between type inference and type annotations:

So, when do you use type inference and type annotations?

In practice, you should always use the type inference as much as possible. You use the type annotation in the following cases:

• When you declare a variable and assign it a value later.
• When you want a variable that can’t be inferred.
• When a function returns the any type, you need to clarify the value.

• Type inference occurs when you initialize variables, set parameter default values, and determine function return types.
• TypeScript uses the best common type algorithm to select the best candidate types that are compatible with all variables.
• TypeScript also uses contextual typing to infer types of variables based on the locations of the variables.

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