Compiler

This is a complete guide to the numerous configuration options in a tsconfig.json file. If you have a specific end result in mind, you can narrow down this list by selecting one of the options below.

noImplicitAny

When there are no type annotations for a variable, TypeScript will assume the type as any. This can cause a wide variety of errors to be missed by the compiler.

function example(s) {
    console.log(s.substring(2))
}
example(42)

If strict or noImplicitAny is set to true, an untyped value will produce a compiler error.

function example(s) {
    console.log(s.substring(2))
}
example(42)

This flag does affect certain cases of dynamic let assignment.

let x
x = 5
x = "six"

The noImplicitAny flag will prevent you from using dynamic types without an explicit declaration.

let x: string | number
x = 5
x = "six"

noImplicitOverride

If set to true, forces class inheritance to use the override keyword when defining a function that overrides a method from a super class.

class Animal {
    makeNoise() {
        console.log('woof meow moo')
    }
}
class Dog extends Animal {
    override makeNoise() {
        console.log('woof woof')
    }
}
class Cat extends Animal {
    makeNoise() {
        console.log('meow meow')
    }
}

noImplicitReturns

Checks all code paths in a function to ensure they return a value.

function() {
}

noImplicitThis

Raises an error when this is used with an implied any type.

noPropertyAccessFromIndexSignature

When this flag is enabled, TypeScript will no longer allow you to use the dot syntax (.) to access fields which may not be defined.

interface GameSettings {
  speed: "fast" | "medium" | "slow"     // Known up-front property  
  [key: string]: string                 // Anything else is a string
}
 
declare const settings: GameSettings
console.log(settings.speed)
console.log(settings.quality)

This setting ensures consistency between accessing a field via the “dot” (obj.key) syntax, and “indexed” (obj["key"]) and the way which the property is declared in the type.

interface Stricter {
    foo: string
    [key: string]: string
}
declare const test: Stricter
console.log(test.foo)
console.log(test.bar)