JavaScript has four different equality comparison operations.

SameValue

It returns true if both operands belong to the same Type and are the same value.

Note: the value of an object is a reference.

You can use this comparison algorithm via Object.is (ECMAScript 6).

Examples:

Object.is(1, 1);            // true
Object.is(+0, -0);          // false
Object.is(NaN, NaN);        // true
Object.is(true, "true");    // false
Object.is(false, 0);        // false
Object.is(null, undefined); // false
Object.is(1, "1");          // false
Object.is([], []);          // false

This algorithm has the properties of an equivalence relation:

• Reflexivity: Object.is(x, x) is true, for any value x
• Symmetry: Object.is(x, y) is true if, and only if, Object.is(y, x) is true, for any values x and y.
• Transitivity: If Object.is(x, y) and Object.is(y, z) are true, then Object.is(x, z) is also true, for any values x, y and z.

SameValueZero

It behaves like SameValue, but considers +0 and -0 to be equal.

You can use this comparison algorithm via Array.prototype.includes (ECMAScript 7).

Examples:

[1].includes(1);            // true
[+0].includes(-0);          // true
[NaN].includes(NaN);        // true
[true].includes("true");    // false
[false].includes(0);        // false
[1].includes("1");          // false
[null].includes(undefined); // false
[[]].includes([]);          // false

This algorithm still has the properties of an equivalence relation:

• Reflexivity: .includes(x) is true, for any value x
• Symmetry: .includes(y) is true if, and only if, [y].includes(x) is true, for any values x and y.
• Transitivity: If .includes(y) and [y].includes(z) are true, then .includes(z) is also true, for any values x, y and z.