Object.freeze after object creation doesn't error

[karlhorky]

TypeScript Version: 3.5.1

Search Terms: Object.freeze, freeze, freezing, properties, object, objects, property

Code

const a = Object.freeze({a: 0});
a.a = 1; // Error: Cannot assign read-only property

const b = {b: 0};
Object.freeze(b);
b.b = 1; // No error?

const c = {c: {d: 0}};
Object.freeze(c.c);
c.c.d = 1; // No error?

Expected behavior:

Cannot assign read-only property errors for the two lines with "No error?" above.

Actual behavior:

No errors

Playground Link: https://www.typescriptlang.org/play/?removeComments=true#code/MYewdgzgLgBAhjAvDA8gIwFYFNhQHQBmATllgF5YAUA3nAFwwAMAvgJQDcAUHHgsgIzsYAemEwAokSIgiDAMJwwYELDgQIASwDmYGCTgATALTgANgE8YAB2lWsRKOc6dQkWGiQxqaBiy7psXEIScio0Dk40PA8BIVEYADkQGHtpIgB+Z1doGGBPamAGagNfZmZ-TBx8YlIKSmA8YAiGhoNPQRExJJSpGXSgA

[karlhorky]

If this is an intentional design decision, it would be a good candidate for documentation on microsoft/TypeScript-Handbook#1059

[dragomirtitian]

Typescript has no way to model the fact that a simple method call changes the type of its parameter.

The only way to model such changes are custom type guards, and these have to be used in some sort of conditional to work:

function freezeAndGuard<T>(p: T): p is Readonly<T> {
    Object.freeze(p);
    return true;
}

(function () {
    const b = {b: 0};
    if(!freezeAndGuard(b)) return;
    b.b = 1; // Err

    const c = {c: {d: 0}};
    if(!freezeAndGuard(c.c)) return;
    c.c.d = 1; // Err
})();

Playground link

[karlhorky]

Thanks for the answer! I suppose we can tag this as "Working as Intended" or similar and close it then?

In any case, good to have this documented at least here.

Maybe also good to add to the handbook, maybe one or multiple of the following:

• under the Object.freeze section on the Utility Types page
• a separate new section for Object.freeze
• the new page for Soundness Full Handbook Page on Soundness TypeScript-Handbook#1059

[karlhorky]

Typescript has no way to model the fact that a simple method call changes the type of its parameter.

Would also be interested: is this a design limitation of TypeScript? Is there a possibility of changing it? It feels like it would not be a small change...

[dragomirtitian]

There were several proposals to implement something like custom type assertion ( #10421, #31512, #31879, #17760, #32130 just to name a few).

It feels small but it actually has big performance implications. From previous comment:

Typescript builds a graph of nodes that can impact the type of variables. This graph includes nodes of constructs that can impact the type of a variable (such as if, switch statemenets, assignments etc). With this change ALL calls would have to be included in this graph (the graph is constructed when type information about the function is not yet available, so there is no way to know that assertString has an impact on CF while other functions don't).

[karlhorky]

Thanks for the detailed explanation! Really helpful to have this all linked together 🙌

[karlhorky]

It feels small

I actually meant to say "not a small change", but I think the not got lost in the text. I've emphasized it.

[jimmywarting]

currently wish i could refactor this:

export class JSXIdentifier {
    readonly type: string;
    readonly name: string;
    constructor(name: string) {
        this.type = JSXSyntax.JSXIdentifier;
        this.name = name;
    }
}

into this:

export class JSXIdentifier {
    type: string;
    name: string;
    constructor(name: string) {
        this.type = JSXSyntax.JSXIdentifier;
        this.name = name;
        Object.freeze(this);
    }
}