===) is only true if the operands are of the same type and the contents match. The more commonly-used abstract comparison (e.g.
==) converts the operands to the same type before making the comparison. For relational abstract comparisons (e.g.,
<=), the operands are first converted to primitives, then to the same type, before comparison.
Strings are compared based on standard lexicographical ordering, using Unicode values.
Features of comparisons:
- Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
- Two numbers are strictly equal when they are numerically equal (have the same number value). NaN is not equal to anything, including NaN. Positive and negative zeros are equal to one another.
- Two Boolean operands are strictly equal if both are
trueor both are
- Two distinct objects are never equal for either strict or abstract comparisons.
- An expression comparing Objects is only true if the operands reference the same Object.
- Null and Undefined Types are strictly equal to themselves and abstractly equal to each other.
x == y
1 == 1 // true "1" == 1 // true 1 == '1' // true 0 == false // true 0 == null // false 0 == undefined // false null == undefined // true
x != y
1 != 2 // true 1 != "1" // false 1 != '1' // false 1 != true // false 0 != false // false
The identity operator returns true if the operands are strictly equal (see above) with no type conversion.
x === y
3 === 3 // true 3 === '3' // false
The non-identity operator returns true if the operands are not equal and/or not of the same type.
x !== y
3 !== '3' // true 4 !== 3 // true
Each of these operators will call the
valueOf() function on each operand before a comparison is made.
The greater than operator returns true if the left operand is greater than the right operand.
x > y
4 > 3 // true
The greater than or equal operator returns true if the left operand is greater than or equal to the right operand.
x >= y
4 >= 3 // true 3 >= 3 // true
The less than operator returns true if the left operand is less than the right operand.
x < y
3 < 4 // true
The less than or equal operator returns true if the left operand is less than or equal to the right operand.
x <= y
3 <= 4 // true
Using the Equality Operators
The standard equality operators (
!=) use the Abstract Equality Comparison Algorithm to compare two operands. If the operands are of different types, it will attempt to convert them to the same type before making the comparison, e.g., in the expression
5 == '5', the string on the right is converted to
Number before the comparison is made.
The strict equality operators (
!==) use the Strict Equality Comparison Algorithm and are intended for performing equality comparisons on operands of the same type. If the operands are of different types, the result is always
5 !== '5'.
Use strict equality operators if the operands must be of a specific type as well as value or if the exact type of the operands is important. Otherwise, use the standard equality operators, which allow you to compare the identity of two operands even if they are not of the same type.
Numbertype value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest
- If one of the operands is
Boolean, the Boolean operand is converted to 1 if it is
trueand +0 if it is
Numbervalue, using the
toStringmethods of the objects. If this attempt to convert the object fails, a runtime error is generated.
- Note that an object is converted into a primitive if, and only if, its comparand is a primitive. If both operands are objects, they're compared as objects, and the equality test is true only if both refer the same object.
// true as both operands are type String (i.e. string primitives): 'foo' === 'foo' var a = new String('foo'); var b = new String('foo'); // false as a and b are type Object and reference different objects a == b // false as a and b are type Object and reference different objects a === b // true as a and 'foo' are of different type and, the Object (a) // is converted to String 'foo' before comparison a == 'foo'
|ECMAScript 3rd Edition (ECMA-262)||Standard||Adds
|ECMAScript 5.1 (ECMA-262)||Standard||Defined in several sections of the specification: Relational Operators, Equality Operators|
|ECMAScript 2015 (6th Edition, ECMA-262)||Standard||Defined in several sections of the specification: Relational Operators, Equality Operators|
|ECMAScript 2017 Draft (ECMA-262)||Draft||Defined in several sections of the specification: Relational Operators, Equality Operators|
|Feature||Chrome||Firefox (Gecko)||Internet Explorer||Opera||Safari|
|Feature||Android||Chrome for Android||Firefox Mobile (Gecko)||IE Mobile||Opera Mobile||Safari Mobile|
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.