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javascriptobjectassign

Primitives missing from properties copied by Object.assign in Javascript

I have a question about Object.assign().

const v1 = 'abc';
const v2 = true;
const v3 = 10;
const v4 = Symbol('foo');

//Primitives will be wrapped to objects
const obj = Object.assign({}, v1, null, v2, undefined, v3, v4); 
// Primitives will be wrapped, null and undefined will be ignored.
// Note, only string wrappers can have own enumerable properties.
console.log(obj); // { "0": "a", "1": "b", "2": "c" }

The output is just { "0": "a", "1": "b", "2": "c" }. I guess this is the result of "abc" being wrapped to an object.

Comment says null and undefined are ignored so they are not in the output.

But what of v2, v3, and v4? They are not present in the output so I wonder what they become if wrapped to objects.

Can you explain why only { "0": "a", "1": "b", "2": "c" } is in the output? I'm guessing this is because wrappers of true and 10 and Symbol('foo') have no own enumerable properties. Am I on the right track?

Thanks.

Solution

  • Yes, your assumption is correct. Strings have enumerable properties while booleans, numbers, and symbols do not. According to MDN:

    The Object.assign() method only copies enumerable and own properties from a source object to a target object.