Functions

Learn about writing functions in Typescript

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Key Concepts

Review core concepts you need to learn to master this subject

TypeScript Parameter Type Annotations

TypeScript Optional Parameter

TypeScript Default Parameters

TypeScript Inferring Return Types

TypeScript Void Return Type

TypeScript Explicit Return Types

TypeScript Parameter Type Annotations

function greet(noun: string) {
  console.log(`Hello, ${noun}!`);
}

greet('World'); // Prints: Hello, World  

greet(2020); // Argument of type 'number' is not assignable to parameter of type 'string'.

Function parameters may be given type annotations with the same syntax as variable declarations – a colon next to the name. The type annotations ensure that the parameters are of the correct type.

Functions

Lesson 1 of 1

1
Introduction
When we declare a function in JavaScript, we often expect it to be invoked with arguments of a certain type. JavaScript does not share our expectations: its type flexibility often allows functions…
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2
Parameter Type Annotations
In TypeScript, function parameters may be given type annotations with the same syntax as variable declarations: a colon next to the name. The type annotations ensure that the parameters are of the …
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3
Optional Parameters
TypeScript normally gives an error if we don’t provide a value for all arguments in a function. This isn’t always desirable; sometimes we’d like to skip providing values. function greet(name: stri…
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4
Default Parameters
If a parameter is assigned a default value, TypeScript will infer the variable type to be the same as the default value’s type. (This is similar to how TypeScript infers the type of an initialized…
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5
Inferring Return Types
TypeScript can also infer the types of values returned by functions. It does this by looking at the types of the values after a function’s return statements. function createGreeting(name: string) …
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6
Explicit Return Types
If we’d like to be explicit about what type a function returns, we can add an explicit type annotation after its closing parenthesis. Here, we use the same syntax as other type annotations, a colon…
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7
Void Return Type
By now, we’ve made a pretty convincing case that type annotations are very useful and should always be used unless there’s a very good reason not to. They make everything neat and aid in understand…
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8
Documenting Functions
TypeScript recognizes JavaScript comment syntax: // This is a single line comment /* This is a multiline comment */ But it’s common in TypeScript to see a third comment style: documentation com…
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9
Review
We learned all about how to use TypeScript to specify the types of function parameters and return types. Now we know how to: * Give type annotations to function parameters. * Deal with type annot…
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