Learning TypeScript

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Enhance your web development skills using type-safe JavaScript. The #1 TypeScript book for JavaScript developers. ✍ by @JoshuaKGoldberg.com; 🖨 by O'Reilly https://learningtypescript.com

Code screenshot:
declare function process(data: number): void;

// @ts-expect-error
// ❌ Include a description after the "@ts-expect-error" directive
// to explain why the @ts-expect - error is necessary.
// The description must be 3 characters or longer.
process("abc");

// @ts-expect-error -- 🤷
// ❌ Include a description after the "@ts-expect-error" directive
// to explain why the @ts-expect - error is necessary.
// The description must be 3 characters or longer.
process("abc");

// @ts-expect-error -- Pending 'process' being fixed to take in string.
process("abc"); // No lint or type error ✅

Code screenshot:
declare function process(data: number): void;

// @ts-expect-error -- Pending 'process' being fixed to take in string.
process("abc"); // No type error ✅

Code screenshot:
// index.js
// @ts-check

/** @type {string} */
let value;

value = 123;
// ❌ Type 'number' is not assignable to type 'string'.

Code screenshot:
// @ts-nocheck

let value: string;

value = 123;
value = true;
value = { a: 'b' };

// Look, no errors! 😱

Code screenshot:
// TODO: data is actually type string; we'll switch this soon
declare function process(data: number): void;

process("abc");
// ❌ Argument of type 'string' is not assignable to parameter of type 'number'.

// @ts-ignore
process("abc"); // No type error ✅

// @ts-ignore
// No type error 🤷
let value: string = "abc";

Code screenshot:
// TODO: data is actually type string; we'll switch this soon
declare function process(data: number): void;

process("abc");
// ❌ Argument of type 'string' is not assignable to parameter of type 'number'.

// @ts-expect-error
process("abc"); // No type error ✅

// @ts-expect-error
// ❌ Unused '@ts-expect-error' directive.
let value: string = "abc";

Code screenshot:
// @ts-expect-error
let value: number = "oops!";