How to compare the equality of two elements Back

In underscore, there is a method called _.isEqual() to judge whether an element is equal to another. Before analyzing this more complicated function, which has been implemented with around hundreds of lines, I would like to discuss when an element is actually equal to another. For example, 1 should be equal to new Number(1), and [1] is also equal to [1] even if they are two different arrays.

For the expression a === b, there are two situations resulting in true:

1. both a and b are primitive types, having the same value at the same time.
2. both a and b are reference types, having the same target to reference.

If this expression returns true, we can say that a is 99% likely equal to the element b, except a special case. According to the article http://wiki.ecmascript.org/doku.php?id=harmony:egal, people think that 0 is actually not equal to -0, but in JavaScript, 0 === -0 will return unexpected true.

/**
 * Identical objects are equal. `0 === -0`, but they aren't identical.
 * See the [Harmony `egal` proposal](http://wiki.ecmascript.org/doku.php?id=harmony:egal).
 */
if (a === b) {
    return a !== 0 || 1 / a === 1 / b;
}

What if one of a and b is null or undefined:

/** A strict comparison is necessary because `null == undefined`. */
if (a == null || b == null) {
    return a === b;
}

If a is a RegExp object, while b is a String, then undercore will convert them into both String to judge whether they are equal:

var a = /a/;
var b = new RegExp("a");

var _a = '' + a; // => /a/
var _b = '' + b; // => /a/

return _a === _b; // => true

In the case when a and b are both Number objects, underscore will handle a special case in which NaN is only equal to NaN but not any other Number objects:

/**
 * `NaN`s are equivalent, but non-reflexive.
 * Object(NaN) is equivalent to NaN
 * If `+a !== +a` returns true, it means that
 * a is NaN, and then just check whether b is
 * also NaN.
 */
if (+a !== +a) {
    return +b !== +b;
}

/**
 * An `egal` comparison is performed for other numeric values.
 * If a is zero, then check `1 / +a === 1 / b` to make sure b is not `-0`
 * If a is not zero, thenjust check `+a === +b`
 */
return +a === 0 ? 1 / +a === 1 / b : +a === +b;

How about Date and Boolean objects?

return +a === +b;

For the Array and Object, underscore has adopted a recursive comparison to judge. When there is a various key-value pair, then return false.

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