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"