Object.freeze after object creation doesn't error

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

[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

[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);
    }
}