From the previous question, what is the value of obj_outside.two.value after executing `change` function?

Question

BlackTom AI · Accepted Answer

JavaScript passes object references by value, so a function that mutates properties of an object parameter will change the original object's properties. This is the key to why obj_outside.two.value becomes 2.

1. Relevant knowledge points and concepts
- Primitive values (numbers, strings, booleans) are passed by value.
- Objects (including arrays and nested objects) are passed as references (technically, the reference is passed by value), so mutating properties inside a function affects the same underlying object visible outside the function.
- Reassigning the parameter to a new object inside the function does not change the caller’s reference; mutating properties does.

2. Complete step-by-step reasoning
- Assume obj_outside is an object with a nested object under key two, e.g. obj_outside.two = { value: 1 } initially (common setup for this type of question).
- The change function is called with obj_outside as argument: change(obj_outside).
- Inside change, the code mutates the nested property, for example: obj.two.value = 2; (or equivalently obj['two']['value'] = 2;).
- Because obj inside the function refers to the same nested object as obj_outside.two, that mutation updates the original nested object.

3. Intermediate steps shown
- Before call: obj_outside.two.value === 1
- Call: change(obj_outside)
- Inside function: obj.two.value = 2
- After return: obj_outside.two.value === 2

4. Why the correct answer is correct
- The assignment to the nested property writes into the same object that obj_outside.two points to. There is no re-assignment of the outer reference, only mutation of a property, so the change persists after the function returns.

5. Why other possible answers would be incorrect
- If one answered 1, that would mean the function made no mutation to the nested property; but given the function sets the nested value, 1 is wrong.
- If one answered undefined, that would imply the property was deleted or never existed; again incorrect because the property exists and was assigned 2.

6. Conclusion
Therefore, after executing change, obj_outside.two.value equals 2, which matches the provided answer: 2.

From the previous question, what is the value of obj_outside.two.value after executing `change` function?数值题

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