Assertions and their cost in TypeScript

as, non-null, and the two escape hatches you should distrust.

const raw: unknown = JSON.parse('{"lang":"ts"}');

const risky = raw as { lang: string };
console.log(risky.lang.toUpperCase()); // trusted, unchecked

const rows: (string | null)[] = ["a", null];
const first = rows[0]!;
console.log(first.length);

function narrow(value: unknown): { lang: string } {
  if (typeof value !== "object" || value === null || !("lang" in value)) {
    throw new TypeError("no lang");
  }
  const { lang } = value as { lang: unknown };
  if (typeof lang !== "string") throw new TypeError("lang is not a string");
  return { lang };
}

console.log(narrow(raw).lang);

const widened = "monokai" as const satisfies string;
const doubled = <T,>(value: T): [T, T] => [value, value];
console.log(widened, doubled(1));

How it works

1. as reinterprets without checking; it can lie.
2. ! asserts non-null and crashes at run time when wrong.
3. A validating function is the honest alternative.

Keywords and builtins used here

• JSON
• T
• as
• const
• function
• if
• return
• string
• throw
• unknown

The run, in numbers

Lines

23

Characters to type

724

Tokens

204

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 136 seconds.

Type this snippet

Step 2 of 2 in The awkward types, step 6 of 23 in Types & annotations.

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