Paul Cowan
Mar 9, 2023 ⋅ 6 min read

A complete guide to const assertions in TypeScript

Paul Cowan Contract software developer.

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5 Replies to "A complete guide to <code>const</code> assertions in TypeScript"

  1. The example in your conclusion is wrong: z and a would not be read-only since those are the keys for nested object. This is currently the behavior of “as const” syntax.

  2. that isn’t true, this is the resultant type:

    let obj: {
    readonly x: 10;
    readonly y: readonly [20, 30];
    readonly z: {
    readonly a: {
    readonly b: 42;
    and this error happens when you try to modify z o a
    Cannot assign to ‘z’ because it is a read-only property.(2540)

  3. The example with redux actions is striking. With interfaces it’s clear and reads nicely, with ‘const’ assertion, it becomes more…implicit and easier to overlook. IMO interfaces are better for this purpose. The goal is not to write maintainable code, not as little code as possible.
    But the purpose of the assertion is clear when it comes to literals.
    Nice article, thanks!

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