Equality comparisons and sameness

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 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 mathematical 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.

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.

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.

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:

js

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.

Math.atan2, Math.ceil, Math.pow, Math.round

In some cases, it's possible for a -0 to be introduced into an expression as a return value of these methods even when no -0 exists as one of the parameters. For example, using Math.pow to raise -Infinity to the power of any negative, odd exponent evaluates to -0. Refer to the documentation for the individual methods.

Math.floor, Math.max, Math.min, Math.sin, Math.sqrt, Math.tan

It's possible to get a -0 return value out of these methods in some cases where a -0 exists as one of the parameters. E.g., Math.min(-0, +0) evaluates to -0. Refer to the documentation for the individual methods.

~, <<, >>

Each of these operators uses the ToInt32 algorithm internally. Since there is only one representation for 0 in the internal 32-bit integer type, -0 will not survive a round trip after an inverse operation. E.g., both Object.is(~~(-0), -0) and Object.is(-0 << 2 >> 2, -0) evaluate to false.

Relying on Object.is when the signedness of zeros is not taken into account can be hazardous. Of course, when the intent is to distinguish between -0 and +0, it does exactly what's desired.

The Object.is specification treats all instances of NaN as the same object. However, since typed arrays are available, we can have distinct floating point representations of NaN which don't behave identically in all contexts. For example:

const f2b = (x) => new Uint8Array(new Float64Array([x]).buffer);
const b2f = (x) => new Float64Array(x.buffer)[0];
// Get a byte representation of NaN
const n = f2b(NaN);
// Change the first bit, which is the sign bit and doesn't matter for NaN
n[0] = 1;
const nan2 = b2f(n);
console.log(nan2); // NaN
console.log(Object.is(nan2, NaN)); // true
console.log(f2b(NaN)); // Uint8Array(8) [0, 0, 0, 0, 0, 0, 248, 127]
console.log(f2b(nan2)); // Uint8Array(8) [1, 0, 0, 0, 0, 0, 248, 127]

• JS Comparison Table by dorey