JS：怪异的对象比较行为

Question

Given this code:

 const name = { name: 'amy' }; function greet(person) { if (person == { name: 'amy' }) { return 'hey amy'; } else { return 'hey there'; } } console.log( greet(name) // 'hey amy' ); console.log( greet({ name:'amy' }) // 'hey there' ); console.log(name == { name: 'amy' }); // true console.log(name === { name: 'amy' }); // false console.log(Object.is(name, { name: 'amy' })); // false 

Why does double-equals comparison return true when using the name variable, but not an object literal?

First, I thought that maybe it's because the objects have the same memory address, but as one can see it's not true.

Moreover, if we turn things upside-down and define the variable inside the function, the comparison returns false ! (It is not shown in the code but you can check it out)

I'm seriously confused and will appreciate the explanation.

EDIT: Here is where I have tested the code. Nodejs and browser console in Chrome give me the regular results, how it should be. So maybe it is something about the interpreter.

Answer 1

You assume that Javascript will perform the comparision with == as you are thinking of it in your mind, but it is not. But as this is a custom object you can't expect Javascript to hand you a custom implementation out of the box. You should implement it yourself.

The only case where this would work is when you use the === operator to check whether the object are the same but by their memory address, thus skipping any custom-object-data-based comparison whatsoever.

Answer 2

The problem here is the use of the word name for your variable.

In a browser, the window object has a name property that must always be a string. If you try to assign something to it that is not a string, it will be converted to one.

Then, when you compare an object with a string, the object will also be converted and the two strings compared. This is why you see it sometimes return true. Example:

 // This will end up being assigned to window.name, and will be converted to a string. var name = {name: "amy"} console.log(name) // [object Object] // Then, when you compare that to another object using ==, the object will also be converted to string. console.log(name == {name: "amy"}) // true, because the string [object Object] is equal to itself. 

Change the name of the variable to anything else, or use let or const , and the problem should disappear:

 // This will end up being assigned to window.other, but isn't converted to string var other = {name: "amy"} console.log(other) // {"name": "amy"} // Now, both are objects, and unless they are the exact same object, comparing two objects is always false. console.log(other == {name: "amy"}) // false, no two distinct objects are ever equal 

 // Using let will not assign this variable to window. let name = {name: "amy"} console.log(name) // {"name": "amy"} // Again, now we compare two distict objects, which always results in false. console.log(name == {name: "amy"}) // false