JavaScript comparison operators: Identity vs. Equality

问题

I\'ve been trying to understand the difference between JavaScript\'s comparison operators: identity and equality. From what I\'ve read, if you check the equality of two objects using ==, JavaScript will try to figure out if they are the same type and, if not, try to get them to that same type. However, === doesn\'t behave in the same manner. So as an example:

var n = \"1\";
console.log(n==1);        // outputs true
console.log(n===1);       // outputs false

So what is the difference between these \"identity\" operators and the regular equality operators? What is the benefit of having both?

Are there differences in performance? I would think that the identity operator would be faster since it doesn\'t do conversion.

Also, how do these differ when it comes to more complex objects, like arrays? Most importantly, what do conventions say about when one should be used over the other, why?

回答1:

The equality operator will attempt to make the data types the same before making the comparison. On the other hand, the identity operator requires both data types to be the same as a prerequisite.

There are quite a few other posts out there similar to this questions. See:

How do the PHP equality (== double equals) and identity (=== triple equals) comparison operators differ? (has a nice comparison chart)
Which equals operator (== vs ===) should be used in JavaScript comparisons?

In practice, the identity operator comes in really handy when you want to be certain that a boolean value is true or false since...

1 == true     => true
true == true  => true
1 === true    => false
true === true => true

回答2:

The difference is that ==, <=, >= and != will do type coercion — for example, force a string to be evaluated as a number. ===, <==, >==, and !== will not do type coercion. They will compare a string to a number, and since the string "1" is not the same as the numeric value 1, the result is false.

Reference is here:
https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators

回答3:

== 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**

回答4:

The reason is that identity or strict operator (===), it compares with no type conversion, that means if both values doesn’t have the same value and the same type, they won’t be considered equal.

A more clear explanation in the following link:

https://medium.com/@ludico8/identity-vs-equality-battle-of-understanding-vs-758d396e922#.hhg396ey9

来源：https://stackoverflow.com/questions/5447024/javascript-comparison-operators-identity-vs-equality

标签

javascript

operators

comparison-operators

equivalence