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Notes Javascript - Equality Comparisons & Sameness



Equality Comparisons & Sameness

JavaScript provides three different value-comparison operations:

Operand/Method Description
=== Strict Equality
== Loose Equality
Object.is() Object Equality

Which operation you choose depends on what sort of comparison you are looking to perform. Briefly:

They correspond to three of four equality algorithms in JavaScript:

Note that the distinction between these all have to do with their handling of primitives; none of them compares whether the parameters are conceptually similar in structure. For any non-primitive objects x and y which have the same structure but are distinct objects themselves, all of the above forms will evaluate to false.



Strict Equality

Strict equality compares two values for equality. Neither value is implicitly converted to some other value before being compared. If the values have different types, the values are considered unequal. If the values have the same type, are not numbers, and have the same value, they're considered equal. Finally, if both values are numbers, they're considered equal if they're both not NaN and are the same value, or if one is +0 and one is -0.

const num = 0; const obj = new String("0"); const str = "0"; console.log(num === num); // true console.log(obj === obj); // true console.log(str === str); // true console.log(num === obj); // false console.log(num === str); // false console.log(obj === str); // false console.log(null === undefined); // false console.log(obj === null); // false console.log(obj === undefined); // false

Strict equality is almost always the correct comparison operation to use. For all values except numbers, it uses the obvious semantics: a value is only equal to itself. For numbers it uses slightly different semantics to gloss over two different edge cases. The first is that floating point zero is either positively or negatively signed. This is useful in representing certain mathematical solutions, but as most situations don't care about the difference between +0 and -0, strict equality treats them as the same value. The second is that floating point includes the concept of a not-a-number value, NaN, to represent the solution to certain ill-defined mathematical problems: negative infinity added to positive infinity, for example. Strict equality treats NaN as unequal to every other value — including itself. (The only case in which (x !== x) is true is when x is NaN.)

Besides ===, strict equality is also used by array index-finding methods including Array.prototype.indexOf(), Array.prototype.lastIndexOf(), TypedArray.prototype.indexOf(), TypedArray.prototype.lastIndexOf(), and case-matching. This means you cannot use indexOf(NaN) to find the index of a NaN value in an array, or use NaN as a case value in a switch statement and make it match anything.

console.log([NaN].indexOf(NaN)); // -1 switch (NaN) { case NaN: console.log("Surprise"); // Nothing is logged }



Loose Equality

Loose equality is symmetric: A == B always has identical semantics to B == A for any values of A and B (except for the order of applied conversions). The behavior for performing loose equality using == is as follows:

  1. If the operands have the same type, they are compared as follows:
    • Object: return true only if both operands reference the same object.
    • String: return true only if both operands have the same characters in the same order.
    • Number: return true only if both operands have the same value. +0 and -0 are treated as the same value. If either operand is NaN, return false; so NaN is never equal to NaN.
    • Boolean: return true only if operands are both true or both false.
    • BigInt: return true only if both operands have the same value.
    • Symbol: return true only if both operands reference the same symbol.
  2. If one of the operands is null or undefined, the other must also be null or undefined to return true. Otherwise return false.
  3. If one of the operands is an object and the other is a primitive, convert the object to a primitive.
  4. At this step, both operands are converted to primitives (one of String, Number, Boolean, Symbol, and BigInt). The rest of the conversion is done case-by-case.
    • If they are of the same type, compare them using step 1.
    • If one of the operands is a Symbol but the other is not, return false.
    • If one of the operands is a Boolean but the other is not, convert the boolean to a number: true is converted to 1, and false is converted to 0. Then compare the two operands loosely again.
    • Number to String: convert the string to a number. Conversion failure results in NaN, which will guarantee the equality to be false.
    • Number to BigInt: compare by their numeric value. If the number is ±Infinity or NaN, return false.
    • String to BigInt: convert the string to a BigInt using the same algorithm as the BigInt() constructor. If conversion fails, return false.

Traditionally, and according to ECMAScript, all primitives and objects are loosely unequal to undefined and null. But most browsers permit a very narrow class of objects (specifically, the document.all object for any page), in some contexts, to act as if they emulate the value undefined. Loose equality is one such context:
null == A and undefined == A evaluate to true if, and only if, A is an object that emulates undefined. In all other cases an object is never loosely equal to undefined or null.

In most cases, using loose equality is discouraged. The result of a comparison using strict equality is easier to predict, and may evaluate more quickly due to the lack of type coercion.

The following example demonstrates loose equality comparisons involving the number primitive 0, the bigint primitive 0n, the string primitive '0', and an object whose toString() value is '0'.

const num = 0; const big = 0n; const str = "0"; const obj = new String("0"); console.log(num == str); // true console.log(big == num); // true console.log(str == big); // true console.log(num == obj); // true console.log(big == obj); // true console.log(str == obj); // true

Loose equality is only used by the == operator.




Same-value equality determines whether two values are functionally identical in all contexts. (This use case demonstrates an instance of the Liskov substitution principle.) One instance occurs when an attempt is made to mutate an immutable property:

// Add an immutable NEGATIVE_ZERO property to the Number constructor. Object.defineProperty(Number, "NEGATIVE_ZERO", { value: -0, writable: false, configurable: false, enumerable: false, }); function attemptMutation(v) { Object.defineProperty(Number, "NEGATIVE_ZERO", { value: v }); }

Object.defineProperty will throw an exception when attempting to change an immutable property, but it does nothing if no actual change is requested. If v is -0, no change has been requested, and no error will be thrown. Internally, when an immutable property is redefined, the newly-specified value is compared against the current value using same-value equality.

Same-value equality is provided by the Object.is method. It's used almost everywhere in the language where a value of equivalent identity is expected.



Same-Value-Zero Equality

Similar to same-value equality, but +0 and -0 are considered equal.

Same-value-zero equality is not exposed as a JavaScript API, but can be implemented with custom code:

function sameValueZero(x, y) { if (typeof x === "number" && typeof y === "number") { // x and y are equal (may be -0 and 0) or they are both NaN return x === y || (x !== x && y !== y); } return x === y; }

Same-value-zero only differs from strict equality by treating NaN as equivalent, and only differs from same-value equality by treating -0 as equivalent to 0. This makes it usually have the most sensible behavior during searching, especially when working with NaN. It's used by Array.prototype.includes(), TypedArray.prototype.includes(), as well as Map and Set methods for comparing key equality.



Comparing Equality Methods

People often compare double equals and triple equals by saying one is an "enhanced" version of the other. For example, double equals could be said as an extended version of triple equals, because the former does everything that the latter does, but with type conversion on its operands — for example, 6 == "6". Alternatively, it can be claimed that double equals is the baseline, and triple equals is an enhanced version, because it requires the two operands to be the same type, so it adds an extra constraint.

However, this way of thinking implies that the equality comparisons form a one-dimensional "spectrum" where "totally strict" lies on one end and "totally loose" lies on the other. This model falls short with Object.is, because it isn't "looser" than double equals or "stricter" than triple equals, nor does it fit somewhere in between (i.e., being both stricter than double equals, but looser than triple equals). We can see from the sameness comparisons table below that this is due to the way that Object.is handles NaN. Notice that if Object.is(NaN, NaN) evaluated to false, we could say that it fits on the loose/strict spectrum as an even stricter form of triple equals, one that distinguishes between -0 and +0. The NaN handling means this is untrue, however. Unfortunately, Object.is has to be thought of in terms of its specific characteristics, rather than its looseness or strictness with regard to the equality operators.

Comparison Table
X Y == === Object.is SameValueZero
undefinedundefined✅ true✅ true✅ true✅ true
nullnull✅ true✅ true✅ true✅ true
truetrue✅ true✅ true✅ true✅ true
falsefalse✅ true✅ true✅ true✅ true
'foo''foo'✅ true✅ true✅ true✅ true
00✅ true✅ true✅ true✅ true
+0-0✅ true✅ true❌ false✅ true
+00✅ true✅ true✅ true✅ true
-00✅ true✅ true❌ false✅ true
0n-0n✅ true✅ true✅ true✅ true
0false✅ true❌ false❌ false❌ false
""false✅ true❌ false❌ false❌ false
""0 ✅ true❌ false❌ false❌ false
'0'0✅ true❌ false❌ false❌ false
'17'17✅ true❌ false❌ false❌ false
[1, 2]'1,2'✅ true❌ false❌ false❌ false
new String('foo')'foo'✅ true❌ false❌ false❌ false
nullundefined✅ true❌ false❌ false❌ false
nullfalse❌ false❌ false❌ false❌ false
undefinedfalse❌ false❌ false❌ false❌ false
{ foo: 'bar' }{ foo: 'bar' }❌ false❌ false❌ false❌ false
new String('foo')new String('foo')❌ false❌ false❌ false❌ false
0null❌ false❌ false❌ false❌ false
0NaN ❌ false❌ false❌ false❌ false
'foo'NaN❌ false❌ false❌ false❌ false
NaNNaN❌ false❌ false✅ true✅ true
When To Use Object.is() vs ===

In general, the only time Object.is's special behavior towards zeros is likely to be of interest is in the pursuit of certain meta-programming schemes, especially regarding property descriptors, when it is desirable for your work to mirror some of the characteristics of Object.defineProperty. If your use case does not require this, it is suggested to avoid Object.is and use === instead. Even if your requirements involve having comparisons between two NaN values evaluate to true, generally it is easier to special-case the NaN checks (using the isNaN method available from previous versions of ECMAScript) than it is to work out how surrounding computations might affect the sign of any zeros you encounter in your comparison.

Here's a non-exhaustive list of built-in methods and operators that might cause a distinction between -0 and +0 to manifest itself in your code:

Unary Negation

Consider the following example:

const stoppingForce = obj.mass * -obj.velocity;

If obj.velocity is 0 (or computes to 0), a -0 is introduced at that place and propagates out into stoppingForce.



Truthyness & Falsyness

In JavaScript, a truthy value is a value that is considered true when encountered in a Boolean context. All values are truthy unless they are defined as falsy. That is, all values are truthy except false, 0, -0, 0n, "", null, undefined, and NaN. JavaScript uses type coercion in Boolean contexts.
Examples of truthy values in JavaScript (which will be coerced to true in boolean contexts, and thus execute the if block):

if (true) if ({}) if ([]) if (42) if ("0") if ("false") if (new Date()) if (-42) if (12n) if (3.14) if (-3.14) if (Infinity) if (-Infinity)

The logical AND operator, &&

If the first operand is truthy, the logical AND operator returns the second operand:

true && "dog" // returns "dog" [] && "dog" // returns "dog"

Logical Or, Truthyness & Short Circuit Evaluation

The logical OR expression is evaluated left to right, it is tested for possible "short-circuit" evaluation using the following rule:

(some truthy expression) || expr is short-circuit evaluated to the truthy expression.

Short circuit means that the expr part above is not evaluated, hence any side effects of doing so do not take effect (e.g., if expr is a function call, the calling never takes place). This happens because the value of the operator is already determined after the evaluation of the first operand. See example:

function A() { console.log("called A"); return false; } function B() { console.log("called B"); return true; } console.log(B() || A()); // Logs "called B" due to the function call, // then logs true (which is the resulting value of the operator)

Operator Precedence

The following expressions might seem equivalent, but they are not, because the && operator is executed before the || operator.

true || false && false; // RETURNS true, BECAUSE && IS EXECUTED FIRST (true || false) && false; // RETURNS false, BECAUSE GROUPING HAS THE HIGHEST PRECEDENCE


A falsy (sometimes written falsey) value is a value that is considered false when encountered in a Boolean context.
JavaScript uses type conversion to coerce any value to a Boolean in contexts that require it, such as conditionals and loops.
The following table provides a complete list of JavaScript falsy values:

Value Type Description
nullNullThe keyword null — the absence of any value.
undefinedUndefinedundefined — the primitive value.
falseBooleanThe keyword false.
NaNNumberNaN — not a number.
0NumberThe Number zero, also including 0.0, 0x0, etc.
-0NumberThe Number negative zero, also including -0.0, -0x0, etc.
0nBigIntThe BigInt zero, also including 0x0n, etc. Note that there is no BigInt negative zero — the negation of 0n is 0n.
""StringEmpty string value, also including '' and ``.
document.allObjectThe only falsy object in JavaScript is the built-in document.all.

The values null and undefined are also nullish.

Examples of falsy values in JavaScript (which are coerced to false in Boolean contexts, and thus bypass the if block):

if (false) { // NOT REACHABLE } if (null) { // NOT REACHABLE } if (undefined) { // NOT REACHABLE } if (0) { // NOT REACHABLE } if (-0) { // NOT REACHABLE } if (0n) { // NOT REACHABLE } if (NaN) { // NOT REACHABLE } if ("") { // NOT REACHABLE }

The Logical AND Operator, &&

If the first object is falsy, it returns that object:

console.log(false && "dog"); // false console.log(0 && "dog"); // 0