You can actually write a JavaScript function that behaves exactly like ==
that should give you some insight into how it behaves.
To show you what I mean here is that function:
// loseEqual() behaves just like `==`
function loseEqual(x, y) {
// notice the function only uses "strict" operators
// like `===` and `!==` to do comparisons
if(typeof y === typeof x) return y === x;
if(typeof y === "function" || typeof x === "function") return false;
// treat null and undefined the same
var xIsNothing = (y === undefined) || (y === null);
var yIsNothing = (x === undefined) || (x === null);
if(xIsNothing || yIsNothing) return (xIsNothing && yIsNothing);
if(typeof x === "object") x = toPrimitive(x);
if(typeof y === "object") y = toPrimitive(y);
if(typeof y === typeof x) return y === x;
// convert x and y into numbers if they are not already use the "+" trick
if(typeof x !== "number") x = +x;
if(typeof y !== "number") y = +y;
return x === y;
}
function toPrimitive(obj) {
var value = obj.valueOf();
if(obj !== value) return value;
return obj.toString();
}
As you can see ==
has a lot of complicated logic for type conversion. Because of that it's hard to predict what result you are going to get.
Here are some examples of some results you wouldn't expect:
Unexpected Truths
[1] == true // returns true
'0' == false // returns true
[] == false // returns true
[[]] == false // returns true
[0] == false // returns true
'\r\n\t' == 0 // returns true
Unexpected Conclusions
// IF an empty string '' is equal to the number zero (0)
'' == 0 // return true
// AND the string zero '0' is equal to the number zero (0)
'0' == 0 // return true
// THEN an empty string must be equal to the string zero '0'
'' == '0' // returns **FALSE**
Objects with Special Functions
// Below are examples of objects that
// implement `valueOf()` and `toString()`
var objTest = {
toString: function() {
return "test";
}
};
var obj100 = {
valueOf: function() {
return 100;
}
};
var objTest100 = {
toString: function() {
return "test";
},
valueOf: function() {
return 100;
}
};
objTest == "test" // returns true
obj100 == 100 // returns true
objTest100 == 100 // returns true
objTest100 == "test" // returns **FALSE**