Equality (==)
The equality operator (==
) checks whether its two operands are equal,
returning a Boolean result.
Unlike the strict equality operator,
it attempts to convert and compare operands that are of different types.
Syntax
x == y
Description
The equality operators (==
and !=
) use
the Abstract Equality Comparison Algorithm
to compare two operands.
This can be roughly summarized as follows:
-
If the operands are both objects, return
true
only if both operands reference the same object. -
If one operand is
null
and the other isundefined
, returntrue
. -
If the operands are of different types, try to convert them to the same type before
comparing:
- When comparing a number to a string, try to convert the string to a numeric value.
-
If one of the operands is a boolean, convert the boolean operand to 1
if it is
true
and +0 if it isfalse
. -
If one of the operands is an object and the other is a number or a string, try
to convert the object to a primitive using the object's
valueOf()
andtoString()
methods.
- If the operands have the same type, they are compared as follows:
-
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 isNaN
, returnfalse
. -
Boolean: return
true
only if operands are bothtrue
or bothfalse
.
-
String: return
The most notable difference between this operator and
the strict equality (===
) operator is
that the strict equality operator does not attempt type conversion.
Instead, the strict equality operator always considers operands
of different types to be different.
Examples
Comparison with no type conversion
1 == 1; // true
"hello" == "hello"; // true
Comparison with type conversion
"1" == 1; // true
1 == "1"; // true
0 == false; // true
0 == null; // false
0 == undefined; // false
0 == !!null; // true, look at Logical NOT operator
0 == !!undefined; // true, look at Logical NOT operator
null == undefined; // true
const number1 = new Number(3);
const number2 = new Number(3);
number1 == 3; // true
number1 == number2; // false
Comparison of objects
const object1 = {"key": "value"}
const object2 = {"key": "value"};
object1 == object2 // false
object2 == object2 // true
Comparing strings and String objects
Note that strings constructed using new String()
are objects. If you
compare one of these with a string literal, the String
object will be
converted to a string literal and the contents will be compared. However, if both
operands are String
objects, then they are compared as objects and must
reference the same object for comparison to succeed:
const string1 = "hello";
const string2 = String("hello");
const string3 = new String("hello");
const string4 = new String("hello");
console.log(string1 == string2); // true
console.log(string1 == string3); // true
console.log(string2 == string3); // true
console.log(string3 == string4); // false
console.log(string4 == string4); // true
Comparing Dates and strings
const d = new Date('December 17, 1995 03:24:00');
const s = d.toString(); // for example: "Sun Dec 17 1995 03:24:00 GMT-0800 (Pacific Standard Time)"
console.log(d == s); //true
Specifications
Specification |
---|
ECMAScript Language Specification # sec-equality-operators |
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