== is the same things as ===, except that == does type conversion
To show you what I mean here is a JavaScript function that behaves exactly like ==:
// 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();
}
This function should help explain why people keep saying you shouldn't use ==.
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 - and that can lead to bugs.
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**
