Javascript object 传入 function 但显然是按值传递

Question

As javascript objects are passesd by reference then why console.log(obj1.a) is showing 1 instead of 7. As function is changing value to 7.

 obj1 = { a: 1 } obj2 = { a: 1 } function c(d) { console.log('d bef ', typeof d) if (typeof d === 'object'){ d = obj2 obj2.a = 7 console.log ('d.a', da) } } c(obj1) console.log(obj1.a)

Answer 1

By passing the data as a parameter, you pass the object value but not the reference, So, you have a new object with a new reference, just scoped for the function.

This is a simpler example.

 // The pet variable have the referance of the { type: 'cat' } object. let pet = { type: 'cat' } // p is a copy of the pet variable but not have the same referance. const changeThePet = (p) => { p = { type: 'dog' } } // here you pass the pet variable changeThePet(pet) console.log(pet)

p will hold the value of the pet but they aren't the same variable or same referance

Answer 2

You seem confused by two different concepts.

In JS, objects ARE passed by references, but variable are not.

What does it means is that:

//WHen you declare your VARIABLE, you assign it a REFERENCE to a new object :
const something = { property : 'foo' };

//Something now is a variable whose value is a REFERENCE to an object, not the object itself.

function success(obj){
    obj.property = 'bar';
}

function fail(obj){
    obj = { property : 'bar' }: 
}
//When you call fail : 

fail(something);
console.log(something.property); //will print foo

//When you call success : 

success(something);
console.log(something.property); //will print bar

In success function above, you access the object by reference through the function argument and you change its property. Because when you call success , the variable something pass its reference to the object by value . So, the argument of success ( obj ) has for value the reference to the object.

in fail , you replace the reference to the object by a reference to another object .

To be clear: the something variable is passed by value . But this value is a reference to an object.

Thus, you can change object properties through this reference, but you can't change the something variable value by assigning a new value to the function's parameter obj .