javascript中==和===操作符的比较

摘引至"JavaScript: The Definitive Guide, 5th Edition " chapter 5 section 4

 

In JavaScript, numbers, strings, and boolean values are compared by value . In this case, two separate values are involved, and the == and === operators check that these two values are identical.

On the other hand, objects, arrays, and functions are compared by reference . This means that two variables are equal only if they refer to the same object.

 

The following rules determine whether two values are identical according to the === operator:

• If the two values have different types, they are not identical.

• If both values are numbers and have the same value, they are identical, unless either or both values are NaN , in which case they are not identical. The NaN value is never identical to any other value, including itself! To check whether a value is NaN , use the global isNaN( ) function.

• If both values are strings and contain exactly the same characters in the same positions, they are identical. If the strings differ in length or content, they are not identical. Note that in some cases, the Unicode standard allows more than one way to encode the same string. For efficiency, however, JavaScript's string comparison compares strictly on a character-by-character basis, and it assumes that all strings have been converted to a "normalized form" before they are compared. 

• If both values are the boolean value true or both are the boolean value false , they are identical.

• If both values refer to the same object, array, or function, they are identical. If they refer to different objects (or arrays or functions) they are not identical, even if both objects have identical properties or both arrays have identical elements.

• If both values are null or both values are undefined , they are identical.

The following rules determine whether two values are equal according to the == operator:

• If the two values have the same type, test them for identity. If the values are identical, they are equal; if they are not identical, they are not equal.

• If the two values do not have the same type, they may still be equal. Use the following rules and type conversions to check for equality:

  • If one value is null and the other is undefined , they are equal.

  • If one value is a number and the other is a string, convert the string to a number and try the comparison again, using the converted value.

  • If either value is TRue , convert it to 1 and try the comparison again. If either value is false , convert it to 0 and try the comparison again.

  • If one value is an object and the other is a number or string, convert the object to a primitive and try the comparison again. An object is converted to a primitive value by either its toString( ) method or its valueOf( ) method. The built-in classes of core JavaScript attempt valueOf( ) conversion before toString( ) conversion, except for the Date class, which performs toString( ) conversion. Objects that are not part of core JavaScript may convert themselves to primitive values in an implementation-defined way.

  • Any other combinations of values are not equal.