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When enabled, this flag causes functions parameters to be checked more correctly.

Here's a basic example with strictFunctionTypes off:

ts
// @strictFunctionTypes: false
function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment
let func: StringOrNumberFunc = fn;
// Unsafe call - will crash
func(10);
// @strictFunctionTypes: false
function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment
let func: StringOrNumberFunc = fn;
// Unsafe call - will crash
func(10);

With strictFunctionTypes on, the error is correctly detected:

ts
// @errors: 2322
function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment is prevented
let func: StringOrNumberFunc = fn;
// @errors: 2322
function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

type StringOrNumberFunc = (ns: string | number) => void;

// Unsafe assignment is prevented
let func: StringOrNumberFunc = fn;

During development of this feature, we discovered a large number of inherently unsafe class hierarchies, including some in the DOM. Because of this, the setting only applies to functions written in function syntax, not to those in method syntax:

ts
type Methodish = {
  func(x: string | number): void;
};

function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

// Ultimately an unsafe assignment, but not detected
const m: Methodish = {
  func: fn,
};
m.func(10);
type Methodish = {
  func(x: string | number): void;
};

function fn(x: string) {
  console.log("Hello, " + x.toLowerCase());
}

// Ultimately an unsafe assignment, but not detected
const m: Methodish = {
  func: fn,
};
m.func(10);