JavaScript provides two equality operators used for comparisons:
- The double equals (
==), also known as the loose equality operator - The triple equals (
===), also known as the strict equality operator
The key difference between the two is that the triple equals (===) operator compares both type and value, whereas the double equals (==) operator uses type coercion so that both operands are of the same type, then compares only the resulting values.
Here are some examples to clear up any confusion:
代码实现
const num = 0;
const str = '0';
const obj = new String(0);
const bool = false;
const undef = undefined;
const nil = null;
console.dir([
num == str, // 0 == 0, true
num == bool, // 0 == 0, true
str == obj, // '0' == '0', true
obj == num, // 0 == 0, true
bool == str, // 0 == 0, true
bool == obj, // 0 == 0, true
bool == nil, // false
undef == nil, // true
undef == bool, // false
]);
console.dir([
num === str, // types don't match, false
num === bool, // types don't match, false
str === obj, // types don't match, false
obj === num, // types don't match, false
bool === str, // types don't match, false
bool === obj, // types don't match, false
bool === nil, // types don't match, false
undef === nil, // types don't match, false
undef === bool, // types don't match, false
]);As you can see from the examples above, using the triple equals (===) operator is far more predictable and intuitive than the double equals (==) operator. Therefore, we recommend you use the triple equals (===) operator for most cases, unless you are entirely certain you want type coercion to be applied to the comparison’s operands.
