TypeScript Literal Types

Introduction

Literals are exact values that are JavaScript primitives.

Types

• String
• Boolean
• Number
• Template Literal String

Type Guards

• TypeScript Literal Type Guard

Inference

TypeScript is not all knowing. The following won’t fly:

function iTakeFoo(foo: 'foo') { }
const test = {
  someProp: 'foo'
};
iTakeFoo(test.someProp); // Error: Argument of type string is not assignable to parameter of type 'foo'

TypeScript infers test to be of type {someProp: string}. There are a couple of ways to fix this:

function iTakeFoo(foo: 'foo') { }
const test = {
  someProp: 'foo' as 'foo'
};
iTakeFoo(test.someProp); // Okay!

function iTakeFoo(foo: 'foo') { }
type Test = {
  someProp: 'foo',
}
const test: Test = { // Annotate - inferred someProp is always === 'foo'
  someProp: 'foo'
};
iTakeFoo(test.someProp); // Okay!