TypeScript 4.6 for JavaScript users

Syntax and binding errors in every JavaScript file

Before 4.6, an unchecked .js file got almost no errors. Now TypeScript reports JavaScript's own syntax and binding errors there too: a block-scoped name declared twice (TS2451), a modifier where JavaScript does not allow one, and the like. They show up in editors that use TypeScript's language service and when tsc reads a JavaScript file. Type errors still need checkJs.

The release notes give one way to silence them in a file: // @ts-nocheck at the top. This site's tests add a second with TypeScript 5.9: the errors appear only when checkJs is left out of the config. An unchecked file with a duplicate const gets TS2451 then, and nothing with "checkJs": false or with // @ts-nocheck.

Destructured discriminated unions

When kind and payload are destructured from a discriminated union into const bindings (or parameters that are never reassigned), checking kind now narrows payload too.

/**
 * @typedef {{ kind: "count", payload: number } | { kind: "label", payload: string }} Action
 */

/** @param {Action} action */
export function describe(action) {
  const { kind, payload } = action;
  if (kind === "count") {
    return payload.toFixed(0);
  }
  return payload.toUpperCase();
}

Parameters that depend on each other

A function typed with a rest parameter that is a union of tuples, (...args: ["count", number] | ["label", string]) => string, narrows its second parameter when you check the first. In JavaScript, put the function type in a @type tag on the variable that holds the function.

/** @type {(...args: ["count", number] | ["label", string]) => string} */
export const format = (kind, value) => {
  if (kind === "count") {
    return value.toFixed(0);
  }
  return value.toUpperCase();
};

format("count", 3);
format("label", "sale");

Mismatched @param names in TypeScript files

In a checked .js file a @param name that matches no parameter was already an error (TS8024). 4.6 adds an editor suggestion for the same mistake in .ts files, where JSDoc carries no types.

/** @param {number} qty */error TS8024: JSDoc '@param' tag has name 'qty', but there is no parameter with that name.
export function double(quantity) {error TS7006: Parameter 'quantity' implicitly has an 'any' type.
  return quantity * 2;
}

target es2022

The es2022 target and lib bring Array.prototype.at(), Object.hasOwn and the cause option of Error.

Code before super()

A subclass with field initializers may now run code before super() in its constructor, as long as that code does not touch this; before 4.6 any code there was an error.