TypeScript subtype All In One

TypeScript subtype All In One

subtype / assignment

	any	unknown	object	void	undefined	null	never
any →		✓	✓	✓	✓	✓	❌
unknown →	✓		❌	❌	❌	❌	❌
object →	✓	✓		❌	❌	❌	❌
void →	✓	✓	❌		❌	❌	❌
undefined →	✓	✓	✅	✓		✅	❌
null →	✓	✓	✅	✅	✅		❌
never →	✓	✓	✓	✓	✓	✓

https://www.typescriptlang.org/docs/handbook/type-compatibility.html#subtype-vs-assignment

demo

// export {};

let n: null = null;
let u: undefined = undefined;

// let test: boolean = null;
// Type 'null' is not assignable to type 'boolean'.(2322)

// let test: boolean = undefined;
// Type 'undefined' is not assignable to type 'boolean'.(2322)


// let test: object = Object;
// let test: object = Array;
// let test: object = Number;
// let test: object = String;

// let test: object = {};
// let test: object = [];
// let test: object = null;
// Type 'null' is not assignable to type 'object'.(2322)
// let test: object = undefined;
// Type 'undefined' is not assignable to type 'object'.(2322)



// let test: unknown = undefined;
// let test: unknown = null;

// let test: any = undefined;
// let test: any = null;

// let test: void = undefined;
// let test: void = null;
// Type 'null' is not assignable to type 'void'.(2322)

// let test: undefined = undefined;
// let test: undefined = null;
// Type 'null' is not assignable to type 'undefined'.(2322)

// let test: null = null;
// let test: null = undefined;
// Type 'undefined' is not assignable to type 'null'.(2322)


// let test: never = undefined;
// Type 'undefined' is not assignable to type 'never'.(2322)
// let test: never = null;
// Type 'null' is not assignable to type 'never'.(2322)
// let test: never = '';
// Type 'string' is not assignable to type 'never'.(2322)

let test: never;


console.log('test =', test);
// Variable 'test' is used before being assigned.(2454)

// let test = 'abc';
// console.log('test =', test);

// 模块声明
export {};

// export {};

let n: null = null;
let u: undefined = undefined;

let test: boolean = null;
// Type 'null' is not assignable to type 'boolean'.(2322)
let test2: boolean = undefined;
// Type 'undefined' is not assignable to type 'boolean'.(2322)

console.log('test =', test);
console.log('test2 =', test2);

// let test = 'abc';
// console.log('test =', test);

// 模块声明
export {};

refs

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