The difference and difference between == and === in JavaScript.

1、 ‘==’

• First check whether the data types of the two operands are the same
• If they are the same, compare the two numbers for equality
• If they are different, first convert the two numbers to the same data type, and then compare
• '==' Only judge the results on both sides of the equal sign, for example: 1 == '1' (compare number 1 with string 1) and the result is true.

2、 ‘===’

• First check whether the data types of the two operands are the same
• If different, return false directly
• If they are the same, compare whether they are equal
• '===' First judge the data types on the left and right sides. If the data types are inconsistent, return false directly, and then judge the values ​​on both sides. For example: 1 === '1' (compare number 1 with string 1) the result is false.

Extended knowledge:

null==undefined //true
null===undefined //false
NaN==NaN //false
NaN===NaN //false
NaN不与任何相等，包括自己本身。