JavaScript Advanced Series—Equality and Comparison in Types

• Equal operator

• Strict equal operator

• Comparison objects

• Conclusion

JavaScript has two ways to determine whether two values ​​are equal.

Equal operator

The equal operator consists of two equal signs: ==

JavaScript is a weakly typed language, which means that the equal operator will be used for comparison Two values ​​are cast.

""           ==   "0"           // false
0            ==   ""            // true
0            ==   "0"           // true
false        ==   "false"       // false
false        ==   "0"           // true
false        ==   undefined     // false
false        ==   null          // false
null         ==   undefined     // true
" \t\r\n"    ==   0             // true

The above table shows forced type conversion, which is the main reason why using == is widely considered to be a bad programming habit. Due to its complex conversion rules, it can lead to problems that are difficult to track down.

In addition, forced type conversion will also cause performance consumption. For example, in order to compare a string with a number, it must be forced to a number in advance.

Strict equal operator

The strict equal operator consists of three equal signs: ===

Unlike ordinary equals Operator, the strict equal operator does not perform type conversion.

""           ===   "0"           // false
0            ===   ""            // false
0            ===   "0"           // false
false        ===   "false"       // false
false        ===   "0"           // false
false        ===   undefined     // false
false        ===   null          // false
null         ===   undefined     // false
" \t\r\n"    ===   0             // false

The above results are clearer and beneficial to code analysis. If the two operand types are different, they are definitely not equal and will help improve performance.

Comparison objects

Although the == and === operators are both equal operators, when one of the operands is an object, the behavior It's different.

{} === {};                   // false
new String('foo') === 'foo'; // false
new Number(10) === 10;       // false
var foo = {};
foo === foo;                 // true

The equal operator here compares not whether the values ​​are equal, but whether they belong to the same identity; that is, only the same instance of the object is considered equal. This is a bit like is in Python and pointer comparison in C.

Note: In order to see the difference between == and === more intuitively, please refer to JavaScript Equality Table

Conclusion

It is strongly recommended to use the strict equal operator. If a type needs to be converted, it should be done explicitly before comparison, rather than using the complex coercion rules of the language itself.

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