Object Spread { ...array } Incorrectly Assignable to number[]

[nadavushka-dev]

🔎 Search Terms

TypeScript does not raise a type error when an object spread { ...array } is assigned to a variable of type number[]. This leads to an unintended assignment where an object is stored in a variable expected to be an array, causing runtime errors when array methods are used.

Reproduction

let foo: number[] = [1, 2];
foo = { ...foo }; // Expected a type error, but TypeScript allows this

console.log(foo); // Outputs: { "0": 1, "1": 2 } (Object, not an Array)
console.log(Array.isArray(foo)); // false

foo.filter(num => num === 2); // Runtime Error: foo.filter is not a function

This issue is not limited to number[] and can be reproduced with other array types as well.

🕗 Version & Regression Information

• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

No response

💻 Code

let foo: number[] = [1, 2];
foo = { ...foo }; // Expected a type error, but TypeScript allows this

console.log(foo); // Outputs: { "0": 1, "1": 2 } (Object, not an Array)
console.log(Array.isArray(foo)); // false

foo.filter(num => num === 2); // Runtime Error: foo.filter is not a function

🙁 Actual behavior

let foo: number[] = [1, 2];
foo = { ...foo }; // Expected a type error, but TypeScript allows this

console.log(foo); // Outputs: { "0": 1, "1": 2 } (Object, not an Array)
console.log(Array.isArray(foo)); // false

foo.filter(num => num === 2); // Runtime Error: foo.filter is not a function

🙂 Expected behavior

TypeScript should enforce stricter type checking in this scenario to prevent objects from being assigned to arrays unless explicitly cast.

Additional information about the issue

Would appreciate insights from the TypeScript team on whether this is intended behavior or a bug.

[MartinJohns]

Duplicate of #60063.

[jcalz]

also #34780

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.